He would have gotten even more training if he tried to hang with the group after getting caught and helped to protect his team leader as best as possible. Not sure if it would have made a difference for Dan, but it would have been better training
How much training does he even need? He raced the entire Vuelta last year & all things considered looked better than he does now.
So the trajectory isn't upwards at all since the Dauphiné crash 2 years ago.
He would never have been able to help Dan Martin much in that situation.
If we take a look at the other riders who were part of the break, Ghebreigzabhier and Valter both lost 4 minutes, Prodhomme and Sanchez 9, Großschartner 13, Roche the same as Froome, and Dina 16. So he certainly wasn't the only one who lost a considerable amount of time in the last 18 km, though I'm not sure for how long they all stayed in the peloton before getting dropped.
Now I don't think Froome will ever be able to ride for the GC in a GT ever again, though I would like for him to get a final crack at a 5th Tour win due to solely nostalgic and historic reasons, and not because I was a fan of him (that ended when he wasn't just a Kenyan entry in the WC ITT any longer). And I fully agree he hasn't been on a steady upward trajectory since his return, but I just think there's a difference between getting dropped on every fairly hard climb, and being in a break with riders who nearly all have been in alright shapes this year, and where they also had to fight a bit, because they never got a huge lead over the peloton.
I also believe from what I have seen, that he also has to measure his powers quite a bit, so that he doesn't overextend himself. Whether that is mostly due to his injury or his age, I can't say.