Tyrael's disobedience is certainly why he and Imperius butted heads. And if Tyrael continued that path up in the High Heavens, it's unlikely that Imperius would have continued to allow Tyrael to continue his shenanigans.
Tyrael avoided one conflict so he could fully focus on another. The rest of the Angiris Council wasn't willing to intervene, but Tyrael was and is. Not only does he not need to be an angel to do so, but staying an angel would have continued to inhibit his goals. It was certainly a risk, but one he felt was worth taking.
How would being an angel inhibit his goals exactly?
Tyreal is already disobedient which means he exhibits some free will, so being an angel isn't crippling his ability to do things on his own.
Being angel gives him many nifty perks like being immortal, being substantially stronger, and being able to fly.
In essence, reverting from being angel to mortal in order to help humans is a lot like a rich person burning all of their money so they can help poor people.
because imperius would have had to make an example out of tyreal, or at least be more successful in it than he was in the cutscene where tyreal shed his armor/angelic powers
having someone like tyreal keep doing what they are doing is undermining imperius' already shakey authority (let us not forget that malthael was the leader of the angiris council until 20 years ago when the worldstone was destroyed, this seat of power is very new to imperius)
so to assert himself as the new leader he would have eventually had to settle the conflict with tyrael and make him fall in line, or have him delt with (that is if tyreal didn't kill him and usurp his position). therefore tyrael became mortal, relinquishing any authority imperius had, and ending their conflict before it reached its climax. allowing him to fight alongside the nephalem - and remember that in d2 he didn't help us like he does in d3, in d2 we saved him from baals chamber, he rezzed our merc in a4 and told us where to go for the act (and handed out 2 skill points), and then portaled us out before destroying the worldstone. in diablo 3 he fights alongside us in acts 2-4
not to mention his actions in d3 mean a lot more because not only does he fight alongside us but he is now mortal, and can be struck down in battle by even the lowest demon/merc (although blizzard makes tyrael un killable lol)