The original concept was that zombies spawning inside walls and triggers being less immersive. I agree with that sentiment. The reason people are defending it is because they think it would make stealth OP.
I think that there are a few things to consider when coming to that conclusion.
#1) What is the relative worth of Agility over other paths such as Fortitue, Perception, Strength, etc
#2) In what ways would it be OP and if so how would one fix it
The first question is relativistic, however, I think that many would agree that the agility tree isn't a bad tree with Parkour being good and from the shadows and hidden strike being your bread and butter. Archery isn't bad for general POI clearing and a silenced pistol can be helpful as well and the SMG offers some well needed faster firing for those oh crap situations. Knives are weak due to bleed being generally weak. Light armor isn't bad as you have an armor path built into Agility, however, you will need the Ghille suit to perfect stealth regardless.
Considering all of that I would still suggest that Strength + Fortitude makes the best combination of perk paths out of all of them and they mesh very well together as compared to Agility and Fortitude or Agility and Strength or Agility and Perception, etc. But we can't say it's a bad path either since as a single path it covers almost entirely what you would want to be doing in the game comnbat-wise with armor options and two decent weapon paths.
The second question is also subjective, but we can look at a few things that combine to make the skillset useful. For starters the way in which zombies detect the player. Stealth is kinda hard to balance as it's a zero sum game. You are either stealthed or your not. The zombies can see you and react or they can't. If the zombies do not see you when crouched and moving at full speed as you cull the herd then that is what makes the path OP as you are nearly immune to danger. If the zombies do see you then archery and pistols/SMG as a path are generally worse in dealing with larger hordes or faster zombies as found in end-game POIs.
To balance I would reduce movement speed bonuses across the board slightly, being much less vulnerable to danger should come with a larger time to clear offset which makes sense. If you are taking your time to not get hit then it should logically take longer to clear POIs and progress in the game. It's a balanced tradeoff since time is valuable in this game to a large degree. I would also re-work sight values. I don't mind zombies not hearing you or smelling you in stealth but if you turn a corner and one is standing in front of you it should activate the zombie. Even with no points in stealth that has been an issue in 7D2D.
In this way you could clear many POIs with little issue outside of some zombies that spawn in areas that would cause them to see you such as being right around a tight turn in a POI. It gives players that want to actually play stealth be able to do so but with the tradeoff that you need general knowledge behind the POI and spawn locations and you have to take your time due to the combination of slower movement speed and reworked line of sight mechanics.
Furthermore, I think once the developers have roaming zombies within POIs (as I belive they stated as much) then it could take the place of many "triggers" and allow for a much more immersive experience where you are actually waiting for the zombie pathing, etc.
I don't think all triggers need to go, I think some are fine, but the over-use of them is what has effectively put quite the damper on Agility and stealth as a whole compared to A20 and to other skill trees in A21. It's fine if you spawn in a few behind the player or in the ceiling, but spawning them right in front of you on the floors when the room previously had nothing and coming out from walls is a bit stupid IMO. I think it's easier to balance a tradeoff with stealth than to force people into removing it completly, especially in the case of end-game POIs where the majority of spawns are trigger based and they send 10-12 zombies at you.