The ps2 was still getting games last year, so 2020

I want bigger titles to stop cross gen now but for some reason games like Far Cry 4 aren't yet but release indies or sport titles for a while longer.

but major development will likely continue till 2015/6 i think

You are seeing this only from an early adopter pov. Some people simply weren't capable of affording a PS360 until recently, that's why  they need support. If a company forgets these users, they will forget the company. And with time, they can move from late to early adopter status. I myself bought a PS1 some months before the PS3 was released (late adopter), but got my PS3 a few years ago and a Vita at more than a year (early). Everyone now wants a PS4 or X1, but if it isn't viable they will be content with a PS360 and upgrade when they can. If you remove the option, they simply won't buy because they can't do it. 

From the big 3, Sony actually is the one that does it right. Their long support for consoles results in an interesting pattern where 25% to 33% of the sales of their consoles happen after the sucessor is out. This is the market they need to reach. It's always the same, they release a final batch of blockbusters and exclusives before cutting support (GOW 2 for PS2, GT6 for PS3). This gen is actually similar to the ones before, we are in a transition time where we have some big next gen titles skipping the old gen and transition titles that comes to both. Next year, probably we will only have next gen games, except by JRPGs and yearly games on PS360. As usual, expect Fifa to keep coming until PS5 or X"whatevernext". And that's how you do it, since late adopters are more interested on Halo 4, GoW 3, Uncharted and other games they missed.

PS1 got some love too, the last game released was FIFA 2005.

Looking at that number of PS2 games, PS3/360 will probably continue for another ~3-4 years. They'll move on at some point, but not now.

They should leave last gen consoles behind as soon as possable

The consoles limitations should be enough.

When should they? Now. When will they? I'll give them a year and a half. Beginning of 2016

That's a good point. With digital distribution the cost of releasing on old hardware is even less. The less taxing arcade games might never stop coming out for ps3/360, at least not until the consoles have been discontinued. That's no guarantee either, the dreamcast got another game in Januari.
http://kotaku.com/a-new-dreamcast-game-was-released-this-week-1511111188

When SHOULD they? When last gen efforts hurt current gen efforts and last gen sales aren't worth the effort by a significant margin. Why would I LIKE THEM to? Right fuckin' now. I want more resources focused on my console. Last gen ports are hogging up too many resources!!