Chelsea will be in loss I guess, there's no team want Jackson except Chelsea release him cheaper than his market value. It's really rare to see old players in Chelsea, Jackson will turn 31 in the next 9 years, 31 years old striker in Premier League mostly can't compete with the young players anymore.
One thing is that all this long term contracts that Chelsea are giving out to players are just a means to beat the FFP rules and nothing more, I don’t see most of this players staying until then, players like Jackson if they fall out of favour or lose form will be sold off immediately even at a lower price, so it doesn’t mean that Chelsea will be keeping him up until that age. But still if Chelsea as of today decide to keep him it will be base on his form and we have seen players over 30 years who still play well as a striker, players like Jamie Vardy are here. cavani was above 30 when he also played for Manchester United so it is about the form of the player and not the age.
I think the fact that Anthony Martial still hasn't found a club is the incompetence of his agent. He hasn't had a good performance in recent seasons. But a few seasons ago he was Manchester United's top scorer. He could get a good contract from an average team.
I agree with you that players agency sometimes contributes to the player’s lack of team or a better club. As for martial he shouldn’t have waited for his contract to expire before looking for a new club. A loan move when he was out of favour at united would have been better but he didn’t bother now every team sees him as being lazy and can’t spend huge wage on him. It is surprising to see Flamengo rejecting him maybe it would be a matter of fee not getting agreed because he was previously on a mouth watering $240k wage at united which i don’t think any club will match now, he should simply just take a huge pay cut like De gea and get a team