You said it yourself. Gear is replaced. Not removed. Also gear is replaced, because the new gear is inherently better. Thus upgraded. Another minor aspect, is with the Transmog system, you do still carry your gear over, in a form. You can always mog on that appearance.
While getting new talents and abilities, which change a spec’s rotation feels good in most cases, it is not the same as borrowed power either. The borrowed power systems implemented in Legion have, in my opinion, caused Blizzard to get stuck in a degrading design circle. They had a big hit with Artifact Weapons. There were issues with it, multi-spec’ing at the start, legendary acquisition ect. But, by and large the community liked them.
Now Blizzard keeps trying to reinvent that system. Adding more and more complex, temporary systems, that require a large amount of work to design and upkeep and patch every expac, to only throw them out at the end. Also in a way, the borrowed power systems dont really feel respectful of the time invested in them by the player. We put a lot of effort into building them up. Then they are effectively erased.
Concerning Alts, with the loot based power progression system, you simply hit level cap, and started the content gear ladder. Look at shadowlands. Which to some cases, is more friendly on alts at this point in the expansion than say legion was at the same patch point. Once you hit cap, you have your 40 renown to hammer out. There is a catch up, but it does take a bit. And you have the conduits which support this system with the soulbinds. Then you have the Torghast/Legendary grind. Depending on what you want to do with your alt, you may have to craft only 1 legendary, or if playing multiple specs, or content, you may have to craft a few. Now these systems are required to play your character fully. They enhance abilities, in a way that they are essential to the characters playsyle. Its a bit of a barrier. It is better than the AP (azerite power) or AP(artifact power) grinds of previous expacs. But it is a lot of stuff, that is just going to get removed.
And now with the Domination gear and the need to re-craft already crafted legendary items, I am pretty much benching my 3 mains. I have invested gold and time into crafting legendary items for each of their 3 specs. In addition, on say my priest, For shadow I have 1 legendary for single target, one for AoE. For Holy, I have one for raid healing, and one for M+. For Disc, I have one. And going for the best stat weights, I crafted them in the slots, which now domination gear will go in. I do not want to have to spend the gold for the blanks, and farm the soul ash, to re-make 5 legendary items for one char. So I leveled one of my LV 50’s to 60 this week, started the renown grind, and that is going to be my main for 9.1. I will craft the legendary for the non domination slot, when they alter the recipe. And I effectively shelf my 3 mains for now.