Hasn't every edition tried to be the best edition ever? Oh, and I thought 5E was meant to be the "Please forgive us for trying to make an MMO into a tabletop game of World of Warcraft" edition.
I suppose not every edition technically tried to be the best ever - 0e, for instance, didn't have to try, because it was the only edition at the time, and 1e & BECMI were probably not trying to out-do eachother, but to ... well, I'm not sure what the point of that was...
5e, though, is very much a 'compromise edition,' meant to heal the rift of the edition war (obviously with mixed results), and is thus meant to be 'for everyone' (every extant D&Der at the time, who cared to respond to surveys, anyway), regardless of prior-edition preference or playstyle.
So it was really aiming for 'broadly acceptable,' and 'evocative of the classic game' more than 'best.' A harder bar to clear, in many ways, but it made it, but for a few stragglers like the Capn...
... and a few mixed metaphors, like that one. ::ugh::
I've created custom monsters in every edition, particularly if I found a cool mini. I find it easier and faster in 5E than in most previous editions.
It's less involved than 3.5, more so than 4e - you can't just plug in level-appropriate numbers, let alone have a balky bit of obsolete-at-release software do it for you - but, still calls for some of the artistry/instinct/experience/whatever we applied back in the day.
I think any 'missing monsters' complaint, at this point, must be more about the validation of seeing it in print, than 'needing' it for use in your own game. If you are a new/inexperienced DM, you aren't aware of the 'missing' beast so don't need it, if you are an old grognard, you're more than capable of whipping it up in your sleep.