Well, this veteran thinks it's about damned time they implemented a more conventional season model. It was fun at first to just hop back in and blow through limited-time events with the same character, but my enthusiasm for that wore off quickly... I even bought the battle pass for the first few go-'rounds, but it wasn't long before all the content was just a slightly remixed repeat of what they had before and very little of it ever posed much of a challenge for a stacked endgame build. So by the fourth or fifth season, I was no longer willing to stick it out for the full reward track, it had become a fairly monotonous chore. This new one is going to be the 15th or 16th season (!), so they no doubt have tons of data pointing to the fact that others have lost interest in that grind in the same way I had. They need to be able to experiment with more significant gameplay alterations to make it interesting again, and it's just not possible to do that without potentially screwing up game-wide balance in unforeseen ways, especially when trying to account for all the people who have put in 100s or 1000s of hours of playtime over the years refining their loadouts to peak efficiency...
So yeah, you're gonna have to start over, but progression will be accelerated and it'll be much more rewarding when you aren't automatically trashing 98% of all loot drops. The campaign actually is not that long (only a couple dozen hours), but they've also made specific mention of having flexibility in how you reach the endgame, so I doubt you're going to be forced to play through it the same way you would with a fresh character previously. Then when the season ends, all your XP and loot is transferred over to your non-seasonal characters, so it's not going to be a case of your time being "wasted" if you participate. It's a big change, sure, but like the other big change (no longer requiring ownership of the expansion to unlock max level and all proper endgame activities), it shows they're willing to make drastic adjustments in the interest of keeping the game more alive for longer. Which is certainly needed for the franchise, now that they've axed the F2P stop-gap game and this is all we have until The Division 3 eventually comes out.