I think it's a mentality thing for f0rest and RpK. Something about how they play CS makes them more resistant to exhaustion and burnout.
Reaction time and decision making shouldn't deteriorate in your 30s unless you live a REALLY unhealthy lifestyle. So the only reason why other pros drop off so much in their 30s has to be burnout.
I think this was more of an issue in the 1.6 days when the pay wasn't as good. Players who wanted to start families had to look for more stable careers.
Now, the pay is so good that there's really no excuse not to devote time to it the same as you would any other job.
Professional athletes only stop when they're physically not able to compete anymore. Roger Federer didn't turn 30 and say "i'm not a child anymore - time to stop earning millions playing tennis and get a 20k per year office job".
The thing is young players are hungry to show up. Older players who already achieved things dont have the same willingness, plus it starts to get boring. It might be a nice job, but doing it for 15-20 years becomes routine
I mean the latter point is not in favour of RpK though, if they switched teams f0rest would seem better and RpK worse. Not even sure who is currently better but being in a succesful team is surely a better enviorment to showcase skill than playing with friends for fun(dignitas).
When Dignitas faced vitality last time f0rest played great but was kinda let down by his team .