My Pentium 4 machine is getting single-digits in vanilla 1.8 and thats with a Geforce 6600. So I'd say this is accurate. 1.6 ran at nearly 60FPS and 1.7 gets around 40FPS. Looks like they are working toward optimizing for the new computers with fast procs and integrated graphics
I consider this pretty inaccurate. The progression of newer versions of the game performing worse on systems is just progress of a game, this is going to happen each year a new game comes out. So drawing that correlation makes little sense.
1.8 actually boosted performance for my work PC which is older, integrated, and far weaker specs. (Or at least the pre-releases did, official release doesn't seem to help much, but I've already established they really screwed up AMD+1.8) Where as my new, annually upgraded CPU+MOBO+GPU took a performance hit in these recent versions. (It still performs more than good enough, but I definitely was hurt more than helped with this crap.)
Also Minecraft doesn't put that much load on graphics processors, its a heavy CPU function. So that part of your assessment doesn't fit, sorry.