Suppose you put both in. You have to engineer the system to be able to handle, smoothly, both TB and RTWP. There are also things in 5e that just don't work as smoothly in RTWP. Because 5e is all about action economy. You have a set number of "actions" you can perform in a given turn. There are spells that can be cast as a Bonus Action instead of a Standard Action. So, the very design of Action Economy would become problematic in RTWP because unless you have a byzantine level of auto-pause, people will end up not using bonus actions.
I mean, if an enemy gets in melee range of one of my Casters, depending on the layout of the area, I'll either do a Leap/Disengage or I'll do a Shove. That gives my breathing room for casting spells. RTWP doesn't make such tactical decisions as easy of a decision to make.
That's why RTWP is not exactly a good thing, because there are classes that are ONLY strong because they've got good action economy.
Rogue for example can move vast distances in a round via Bonus Action. Far more than anyone else. This gives them a huge advantage in terms of mobility.