Keep in mind that Key being upgraded is a design to REWARD players who over gear or over cheese the content.
People will always Over gear or over cheese things. It is human nature to find the easy way to do things and it is the nature of RPGS to over gear.
Case 1:
Have mythics without timers:
Players will either outgear them(Make it faster) or players will cheese them(Make it faster)
We always have....
Case 2:
Have mythics that scale with IL:
Players will never outgear them(Definition) But will cheese them(Make it faster)
We always have...this is how the runs in MOP were done that scaled with us.
Case 3:
Have the timers but no penalty for timer
Players will either outgear them(Make it faster) or players will cheese them(Make it faster)
Case 4Current situation)
Have a timer but no gear penalty(Just no upgrade)
Players will either make the timer or not:
If they make the time then either they outgear it and thus need an upgrade to be interesting or they cheese it and need an upgrade to make it interesting
If they dont make timer then either they dont outgear it(And should keep running until they do) or they cant yet cheese it(And need to figure out better strats to upgrade)
Case 4 seems the best