The designers had a bias to old school gaming. However it was the survey results that tilted the game by cutting good ideas.I mean, that means it was on purpose.
They just didn't specifically set out to do that. They clearly had vision for what 5e would be, more than once, but steadily ceded more and more and more ground over time. That's why they surrendered the playtest Sorcerer/Warlock. That's why they abandoned the "tactical combat module". That's why the "Fighter Warlord" and martial healing were total vaporware despite Mearls giving a full-throated defenses of the latter on Twitter.
I don't know if I was the person who called it an "apology edition" first. But I've definitely called it that many times, and there's a very good reason for it. It is shackled to a bizarre hybrid of 3rd edition structure and old school concepts, not really achieving what either side particularly wanted. They took no action which might offend the delicate sensibilities of the angry 3e and 2e edition warriors who were busy with their victory dance.
Overall from 2nd edition to 3rd edition to 4th edition to 5th edition all we've been doing is given the fan base less and less restrictions so it's no surprise that they voted for less restriction and end up with a situation where their low restriction spells can control the hell out of the boss monsters.
The mistake the designers did is not sneak in to counterweights for these restrictions.
The counterweights can be hidden.
I bet you 75% of the people who bought 5.5 don't realize that most mid to high level legendary monsters have Proficiency in three saving throws.
"The players will vote out the fun of the game.
Designers have to sneak it back in."