The reason they would encourage non seaonal is to have more active players, in the game world for active group events as well as obvious BP etc carrots.
Legacy items suck and could have been less harsh had the game had a chance to be Dev and tested before launch. But they probably ran out of time and POE 2 and whatever else, pushed it out.
I used to play Destiney 2 and it went through a similar thing. FPS looter shooter.
So I would guess things will calm down on that front eventually. Once things are sorted and more final and foundational.
If you hunted good items, they will be enough to grind out the new set more or less and respec a legacy character, or like I do just keep them for the fun builds to run now and then, and focus on a new concept and on seasonal, grind, put it down and come back next season for a fresh start, and this feeds the eternal for my muck around fun play.
That said, I wish across the board eternal or seasonal we had options for fresh starts etc.
Since if a season is not to my taste I could otherwise try the new update and play a fresh eternal.
That is how i view things. I get where you are coming from though and it does sting, which after playing many games that have done this like Destiney 2, I learnt my lessons on Live service model games, so you learn to not overly invest time, until things stabilise.
Trust me in the peak of Destiney 2 I went after gear that I knew was getting legacy treatment etc and all of that, I haven’t played the game in years now lol. So do what makes you happy but keep that in mind with live service and how you play them.
The downside to them is what i discussed versus how games used to roll out, the advantage is fresh content and reworks and live changes and updates, which can be a blessing and a curse 
