Do you believe Is this a trend going forward or not? That after long contracts top players will opt for free agency ?
This is trend is well entrenched by now IMO. Wenger predicted this when he was managing Arsenal, and Perez has been handing out 6+ year contracts for more than half a decade by now. The risk with going Bosman is that a player picks up a long term injury in the last 6 months of their contract, and then they're left out to dry. But somehow this doesn't happen too frequently for the top players. Valdes got screwed over by Monaco after he got injured around the end of his days at Barsa. Similar thing happened to Gundogan as well, I think. He ran out his Dortmund contract, but wasn't able to find a club in the summer due to his injury record. He re-signed with Dortmund, proved his fitness and moved to City soon.
If you consider the free transfers as the ultimate show of player/agent power in this day, then Raiola seems to have invented another subset of such power transfers where the players don't have to wait till the end of the contract.
Pogba to United, Haland to Dortmund, Mkhi to United are these types of transfers. The agent orchestrated and planned the players transfers well into the future, making sure to have buy-out clauses put in their first contracts. He has made big bucks for himself and the player, while still providing value (monetary or sporting) to the selling club.
Who do you think will be best placed to exploit it this summer and why?
Raiola and his players - He will make big bucks whenever Haland moves. He'll make money on Pogba as well, but he won't be maximizing the transfer value due to his performances. Zahavi seems to have woken from this slumber over the past few years, and has been working to move around Bayern players on a Bosman.