>>714151406Powercreep varies. They mostly release really strong top meta units on anniversaries, other new units vary greatly. They occasionally release special "favorite items" that can buff completely useless old units to top meta relevance. Also recently they've been releasing units with extremely niche support kits that only really apply to 1-2 old units but turn those old units to top meta relevance again. Also most units tend to be good only in specific niche, like story/bosses/PvP. Only extremely few units are great everywhere.
At the end of the day for story purposes meta doesn't matter much. Top units allow you to push the story faster but eventually you'll hit the daily login and building up combat power-grind anyway. Bad units need to be like 10-15% of the required power, a full meta team might cope with 20-30% deficit but beyond that you start doing 1 damage and instantly dying. So if you're willing to wait longer to build up your units you can technically clear the story with any random team.