Second Life's existing catalog of third party content is a massive barrier to entry for possible competitors. They are extremely nervous about breaking it. For example, they kept the invisiprim hack working for a long long time after they introduced alpha layers, because people had existing avatars that depended on it. I don't see them basically wiping out the amount of existing content you're suggesting.
If they were smart about it, they wouldn't have to wipe out content to the extent you're suggesting. I don't think they should suddenly drop a restrictive cap on avatar attachments all of a sudden and out of nowhere. I have a very low opinion of the Lab, but even I don't think LL is that stupid.
First, and these are all things I've said repeatedly over the years, any cap LL introduced would not have to be all that restrictive. I've always said LL only needs to curb the worst habits. They don't have to hold SL content creators to the same standards as professional game artists, just nudge content creators gently, yet firmly, in the right direction.
Second, I don't think LL should rush into any sort of new restrictions on avatar attachments. They should ease them in in a way that doesn't disrupt the community the way you suggested would. They could easily introduce a new, server side, version of ARC where the information is easily viewable but no caps are yet enforced. They could tell people "Hey, this is what we have planned, but it's not coming for a while yet." This would give content creators time to update existing content (which would be relatively easy for content creators to do in most cases, unless they just use uploaded game/asset site rips and don't actually know ANYTHING about actually creating content, aka: content pirates/copybotters/thieves whatever you want to call them) . And when I say "giving content creators time" I'm talking YEARS. Unless LL has immediate plans to shutter SL they can afford to take the long view on this.
Finally, and this is very important yet frequently overlooked, older content typically is not the problem because as time passes, fewer and fewer people use that content. To such an extent that LL could grandfather in content made before a specific date. If they wanted to go really long term with this, they could grandfather in ALL existing content and only apply the new restraints to content going forward. Time has always been on LL's side here. Like you pointed out, they eventually phased out the invisiprim hack. Not right away, but eventually they did. And people don't use invisiprims anymore. Eventually, content gets phased out naturally.
And on top of that, not all old content is bad. I've seen people walking around in old Luskwood and Uchi Desmoulins avatars (mostly because they're freebies these days) who were still way below my ARC caps, and well within acceptable range for both polygon counts and texture use.
I get why people jump this kneejerk conclusion that adding in any sort of restrictions now would cause the content apocalypse, but it's not true. It's never been true. And you only need to spend a few minutes thinking about it to realize how much of a non-issue it really is.
