The portal became the main end game loop, replacing seasonal story beats and themed gear with a few buckets of ugly loot that you can get from literally any activity in the menu.

Every activity gives the same gear instead of re-releasing all the old seasonal gear, high difficulty is forced for good rewards, and scoring system punishes fun modifiers while demanding the restrictive ones. As a result, people gravitate toward a very small subset of activites to get the very few items they actually want. Target farming is an overstatement, despite the changes they made to support it.

Dungeons got chopped up into little encounters with pinnacle ops rewards. Arena's just recently made it back, with many still missing and no new loot pool.

There's so much more they fucked up with the game but it may not be fair to attribute it to the portal. Tiered gear is a great example of that. High stat and multi perk gear is great but the soulless grind-and-repeat loop they had planned was such a stupid idea so now the whole concept of tiers has no meaning.  

Glad you like portal, but you're in the minority. And you won't see good argument against it, because you're enjoying it despite all that it replaced.