altcoin early adopting devs and their supporters would rather have everyone switch to their "new, improved" altcoin for maximum individual profit at the expense of Bitcoiners losing value from having to be later adopters. working to improve Bitcoin isn't sexy or motivational enough for most of them despite the fact that any BTC holdings they might have would likely increase in value from their contributions to the Bitcoin protocol. most of them probably don't even have BTC hence there is no motivation for them to become later Bitcoin adopters (even though it is still EARLY).
This is completely wrong in a large number (most probably a majority) of cases. Altcoin developers have generally been motivated by profit, either BTC or fiat profit, but probably BTC profit more often. This pretty much has to be true for the majority of altcoins that have no significant technological value at all (what else would the motivation be). These developers are certainly not people who don't have BTC or late Bitcoin adopters (of course such a thing doesn't exist now).
You really have to ask more about what is the motivation for altcoin
investors than developers. For most of the developers, the motivation flows pretty obviously from that. And again in that case I think you are looking at a group of people (the investors) who for the most part just seek BTC/fiat profit.
It is only in very rare cases do altcoin investors or developers think their technology has merit or their coin could possibly be the next big thing (either bigger than Bitcoin or a coexist with the meaningful value). You can probably count those on one hand. Maybe even with missing digits.
Given the relative immaturity and near-total lack of adoption that cryptocurrencies have seen generally I don't really see anything wrong with having a few contenders at this point. If we ever get to the point where a cryptocurrency is widely adopted and you can rationally say it represents something even vaguely resembling a global ledger, then overturning it would likely be very destructive. That doesn't exist now. The closest thing we have to a global ledger is fiat, and overturning that would indeed be very destructive (using the term in a morally neutral sense as in "creative destruction")