I don't see why we would even want them to stop it. There's no possible way SL could make an in-world building toolset that would even marginally approach the capabilities or quality of what you can make using modern 3D creation software.
I don't want to put down the talents of prim-builders or the creativity it took to work within the limitations of the prim system and build awesome looking stuff with that tool. But let's level here - for stuff of equal "visual fidelity", mesh done competently looks better and is less resource-intensive, sometimes by loads - and that's not even getting into things like materials and rigging.
I think with things like estate Windlight, experience keys, and well the whole scripting system, there's still plenty of in-world creation going on in SL.
Well I hate to be the only one to disagree with this, but I do, fiercely. What I found when I joined in 2004 was a very collaborative world, one where people would share their skills, their builds, their techniques. I still find people in SL willing to spend time and money to help others to fulfil their creative vision, lending land, providing support, giving people time and space and attention when they have nothing to gain themselves except the satisfaction of helping someone else.
I adore building with prims in Second Life, and hate Blender and Maya with a passion. What I will do, for hours, for enjoyment in SL has me ready to kill my computer with a hammer after only 20 minutes, trying to build in blender. In SL I feel free and able to cobble prims together easily, however much I try, I will never have the same enjoyment from building in a 3D design programme, and more than that, it can never be the collaborative exercise that co-building in SL could be. When we built Numbakulla in SL, there were a group of 10 people, which reduced to about six people in the course of the project, building together, sparking ideas from each other and sharing the space in a way which is simply not possible if you take all the creation outside SL. Yes, you can share mood boards, and design ideas, but it isn't natural and organic in the way that working in SL with prim building tools was.
If LL had put their effort into making an in-world mesh-making tool, I think that would have been a far better use of the time and expertise at that disposal, frankly. Like others, I have watched the build of Sansar with some scepticism: SL needing good graphics cards and a relatively high-end machine was enough of an entry barrier to make many projects die during the brainstorming session with companies, and particularly health services; adding in VR headsets to the mix wasn't likely to make it better or easier to sell.
Constantly seeking a way to find a new audience and ignoring the audience you have is not the pathway to success. Rod Humble seemed locked into the idea of SL as a game, and that is a significant misunderstanding of most residents' way of using the platform. He thought that attracting gamers to SL would be a path to success, and thus we have pathfinding and experiences, but the restrictions and cost of land in SL is a formula that won't work economically for gaming, which I would think would be obvious with very few calculations: maximum 50 players in a sim at a time, sim costs $300 a month to maintain, plus other running costs, development costs, servers for backends etc... how can that ever make any sense for anyone for a game? Sansar wasn't the answer either, because there are better alternatives already available. It's only looking at SL as a virtual world that you can begin to understand who the audience is, and what they are doing with it.
My major complaint over the past 15 years has always been the lack of management in the company, not the wrong management. The loss of the Mentors is something I go back to again and again... the poor running and policing of the events list, the lack of any vision for those things, which have been allowed to die quietly in a corner. From my own observations, the programmers and administrators at LL do not run the show, they are subject to the foibles of the management and whatever they propose. It seemed to me that whatever stupid idea they received from on high, they ran with it, and that making any argument against those things was seen as mutiny. I have never lived or worked in America, but I am guessing that the fact that you can be fired at any time for anything makes people compliant and unlikely to point out the flaws in the plans presented by management. That's certainly how it seemed to me, anyway. When faced with a leader who demanded that we make a cel-shaded game in SL within a few weeks, no-one seemed to have the gumption to tell him, that's not a good use of SL.
(Edited to add: I wanted to elaborate on this: I mean that once you get below the decision makers, there seems to be little appetite for management in the way I understand it as a UK-based person: it seemed to me that there was just an anxious rush to put into practice whatever came down from on high. The comment about mentors is because LL simply closed the programme down instead of deciding how to manage the increase in numbers and the chatter on the mentor channels. With events, they have rules which are generally ignored and which are blatantly and flagrantly broken by many of the sex-based sims, which seem like a front for on-demand online sex services now.)
I still believe that they have lost something significant by switching to mesh uploads as the main source of new content, rather than in-world construction tools, because of the loss of collaboration and community. I guess they are seeing the new lands of Bellessaria as a success, as people are clamouring for more, but the significant advantage of double prims and an actual garden space cannot be underestimated. Had they put the same housing that they have on the old Linden homes sims on new plots with double prims and garden spaces, I think they would have been equally popular. This would be an option with the old sims, as many of the old Linden homes seem to be empty. They could simply remove the empty ones and add gardens and prims to those which are occupied. I don't expect them to do that though.
PS I'm personally very sorry to see Nyx go, as I worked with him on Linden Realms and he seemed far more intelligent and grounded than most of the Lindens I had contact with. I hope he finds somewhere which appreciates him better.