I actually think "more enemies = more actions" makes sense, up to a limit (and there is a limit of 3 for legendary actions).
If you have a spear and a barrel of 2 fish, how many times could you hit a fish in 10 seconds? Consider that if there were 30 fish in the barrel, you would be able to spear many more fish than if there were only 2.
This is only a real world example, but using action movie logic it becomes even more plausible. A giant wildly swing his sword will be making more "attacks" when surrounded by enemies than when facing only one.
Of course these legendary creatures can concentrate all of their attacks on a single creature, which may seem to make less sense. I still think it is OK because realistically a party of adventurers would make larger delays (on the scale of seconds) when coordinating their actions with one another (wait until I drink this potion, then cast spell X on me; or wait until I kill this monster, then cut the rope bridge) whereas a berseker just whaling on an enemy by himself is going to act faster.
The only downside I see to the legendary system is that if it is played as written then as soon as creature acts out of turn the PCs will say, "oh snap, boss monster!" and realize that they are fighting a monster of a distinctly different caliber. This may be obvious, in the case of an Adult Red Dragon or whatever, and some people might not be bothered at all. I suppose that a DM could periodically make a lesser monster- say, a troll- split its multiattack up between player turns, just to make combat in general seem more natural (and also to make the troll seem more epic when up against low level PCs)
All in all I like the system, and also the new Lair actions.
And of course a vampire can't travel faster overland (on a scale of miles) when it chases 3 PCs than when it chases 1 PC. The rules aren't written to give anyone this impression. They are clearly written to cover combat scenarios only.