There's an additional limit to "the gm can send more dragons" that I don't think I've seen come up in the thread. Going with higher CR monsters & higher CR encounters carries a laundry list of issues with each. Simply adding more lower CR monsters as bounded accuracy was intended to do ignores the fact that the GM is a human not an computer AI routine:
- The Players & GM only have so much table time & each additional monster has a turn that extends how long combat takes. This one is minor but notable when it comes to 5e combat feeling like an endless slog
- The GM needs to track each monster in the initiative order, if a group of 4-5 need 2-3+ monsters per player to be challenged under the design of the game there is a serious problem
- Wotc/Wizkids doesn't make minis with enough specificity to differentiate so you wind up with zombie 1 through zombie 15+
- Digital tabletops can help with colored rings & such but when you need to move beyond primary colors into multiple variant shades you hit "which green one?... no that's teal" type problems