This is why Synthesist, although significantly weaker than regular Summoner, is so often called "overpowered"
You will also get a lot of people like myself who really disagree with the idea that it's a lot weaker.
No shared item slot issues, more feats, but most importantly it's not an Eidolon shackled to a squishy Summoner, the Syntheist is unkillable and outside of very niche things, anything that challenges the Synthesist will make paste out of the rest of the party.
Although this is based on personal experience of having never used the aging bonuses and penalties, immortality has always been more of an RP award, like a big house or a title, I'd never require a player take it in place of an actual mechanical ability.
But I'll not be that guy and help out as not every GM will share my thinking.
There's a Bard masterpiece, Song of extinction. Every person you kill with it extends your life by a year so it's incredibly easy to get immortality with that.
I don't see the issue here. The high level martials are doing there main goal well and not using some obscure cheese to do it.
If this had been a thread about how the casters where dominating everything people would just say "well that's high level pathfinder"
I also guess that there is some focused vision here. There are surely lots of scenarios where even with flying kick and pounce martials will only get 1 attack or not attacks at all and the focus is on those rounds where everything worked out perfectly for them.