You cannot have progression that does not render old gear and content obsolete.

People get bored doing new content for sideways "upgrades" a lot faster than they get bored doing new content for gear that actually makes their character stronger.

We have already seen this, if in a small way. It took the duty roulette to really get people into low level dungeons to help out people trying to do them for the first time (or for exp) because those low level dungeons do not offer anything really useful to a higher level character. Why would a level 40, or a level 50 i97 want to queue for Sastasha, when the exp is either tiny (or useless), and the gear is useless? They didn't want to, until the Low Level Duty Roulette. Similarly, people don't want to continue to farm AK/WP for gear when they can farm BCoB, SCoB, etc. for higher ilevel gear. In effect, the lower level and lower ilevel content is obsoleted by higher level and higher ilevel content/rewards.