Absolutely, a 3-4 is a fantastic run stuffing defense. My argument is simply that its difficult to find enough players to pull it off to be effective in both the run and pass game. A lot of these 3-4 teams are giving up a lot of yardage via the pass. I might be missing one but here are the 3-4 defenses that were top 10 in run defense in 2024...
Baltimore - #1 against the run, but # 31 against the pass
Pittsburgh - #6 run, #25 pass
Denver - #3 run, #19 pass
You have to have some of these big guys on the interior who can also attack the passer at a high rate which are very difficult to find, especially if you're not using #1 round picks to do so. If you are in your base defense and a team is throwing it on you then you also have to think about which of my EDGE pass rush specialist am I keeping in coverage? Are you rushing both guys every time, essentially creating a 100% blitz percentage leaving your secondary short handed? You dropping a Mican Parsons into coverage on occasion giving him fewer chances at the QB? This is where it always gets tricky to me and in addition to the 'teams are in nickel 70% of the time anyways' argument which just makes me think the 4-3 vs 3-4 argument isn't really worth the time. Both work, both have pros & cons, but I think if the Cowboys would just address the roster with a couple stronger run defenders instead of being all in on the pass defense I doubt we are that worried about it in general.