Are Monitor/Subs More Room Flexible?

I would agree with your statement generally speaking with two comments:
- more flexible yes but not necessarily better sounding due to the need for quasi-perfect integration that is often under-estimated
- if you can bi-amp and modify the power delivered to the woofers vs. the power going to medium and tweeter (e.g. Linar 10 5.1ch integrated), you can have a very flexible solution without a sub (although you virtually mess with the "cross-over" and may affect the integration as well)

I do not disagree with Vladimir that Rel subs have had such an audiophile sonic response that their integration is never boomy nor gross...yet it is possible to create a small dip, Rel or not, depending on your Main speakers cross-over slope and Rel's.
I think the other thing is to know your room response and try both. My older audio room was a living nightmare for full-range speakers and I never managed to get it right, way too many nodes and stranding waves where speakers made sense. Monitors and sub were much better most of the time.