As someone who hasn't downloaded a Vita or PS3 plus game in years, hopefully sooner than later. I'd rather see that money put towards more PS4 games or more AAA PS4 games or PSVR. I'm sure they have internal data showing a enough people still download them to keep releasing them.
What money? The money it costs to keep ROMs on a server? That's pennies on a dollar compared to AAA game development. Sony is making far more money keeping millions of PS+ subscribers and gamers in its ecosystem than it is hoping to sell a few million copies of games that cost hundreds of millions in development, publishing and advertising. If you want Sony to keep bankrolling more games, you should want this service to stay available for as many subscribers as possible.
According to this post, the developers of the PS+ games don't get a cent from the games being downloaded via PS+. Sony pays the developers to put their games on PS+. The fewer games available on the IGC, the more the developers would get paid and the better the IGC will be.
Personally, I think 2019 would be the cut-off year for the PS Vita and maybe a year after for the PS3. I think the number of people playing on those systems will not be massively significant by then.