There is no comparison between the 5400/5600 series tractor and the new 4600 series. Different class of machine and about 3 pay scales in price. The 5600 series is a true mid size tractor with full features while the 4600 is a utility machine, more basic. Though there is a market for a basic tractor in the 60-90hp market with no power shuttle & no frills, there is a bigger need for a mid-size, mid-optioned machine. Kubota, Deere and CNH all have one and they do well with it. Agco has needed one since the old 4200 series was replaced.
There are HUGE things happening in Agco's under 100hp lineup in the coming years. It has way more to do with tier-4 emmisions than issues with the 2600HD series. The old 200, 400 and 500 series had thier growing pains as well. I had a 481 that we put a few thousand hours on. For the price you couldn't beat those machines. You were buying a new tractor for the price of a used one. If they were worried about warranty claims they would replace the Indian built 2600's.
Every manufacturer will have big changes coming and some will just go away.