The answer is always found between the chair and the keyboard. The people designing the game also sit there.
My main point was to compare the benefit of timers vs no timer. You only argued that you personally have not a lot of leavers, although that doesn't tell me how many that are. If you have one leaver every 10 dungeons I would still consider that to be many when you play a lot of M+ per week, especially if you go for the 3rd Vault slot on multiple characters. That would be 2 leavers on 2 characters per week on average.
What is your counter argument against having no timers in place? Brute force would be such an issue for you? I already proposed a couple solutions for that to discourage this heal/tank stack behaviour. If you enjoy the potential esports aspect of timers (MDI and such) then I can't really argue against that, because that is a subjective benefit of timers that are driven by competition and as such are viable points.
Like, if somebody tells me he likes timers because he likes to increase his r.io score then I accept that as a good point. Some people derive fun out of this artificial system and that's fine to me.