Casper's competition was Hippo. But since Hippo went out of business, there's not really anything else on the grid which comes even near its functionality.
And as we know Linden Lab has the tendency to buy stuff, which tends to get too big/important for the grid to fail, like XStreetSL, so - even tougher to make a new competition again. It's another welcome income source for LL, and one area less where normal scripters now can shine.
Also reaching the level of customer experience which Casper offers is not done easily, since it is a really broad system nowadays.
Of course people could use other vendors, but if these do rely on external stuff, it's also an uncertain bet that this supplier will be still around in 5 years or more. It happened enough times some vendor suppliers vanished, rendering their systems pretty much useless. Casper now on the other hand is here to stay as long as SL exists.