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.
^According to that post up there^, Sony spends buttloads of money for PS+ games that rival the cost of AAA game development, which has the unfortunate side effect of limiting the amount of PS4 games it can provide consumers. Sony isn't making money hand over foot with PS+, and keeping the service alive for PS3 and Vita is a humoungous albatross around its neck, but the world's leading scientific minds can't quite figure out why Sony doesn't just end it for those old consoles. BREAKING NEWS!