That bolded bit misses the problem even though you mentioned it in the underlined bit. As a DM in past editions you had more flexibility & dials you could crank, not so in 5e. take the 3.5 MM where you had a cr5 mummy & cr15 mummy lord along with cr4 allip xr9 bodak cd12 devourer cr2 ghost cr2 lacedon cr4 ghast cr almost anything lich template cr 14 mohrg cr3 shadow cr9 greater shadow cr7 specter cr almost anything vampire template cr4 vampire spawn cr4 wight cr5 wraith cr16 dread wraith nine different CRs of zombie ranging from cr1/2 to cr6 nine different CRs of skeleton ranging from cr1/3 to cr8.. etcI don't see the need to split off a side discussion about rules comparisons. I can sum up my case pretty adequately here.
3e went a long way towards unifying D&D by unifying behind the basic "Most things you do are going to have you roll a D20 and add something and you are going to try to equal or beat a DC". This concept is actually separate from the idea of blank canvas space, but it's an important concept of why I say that 5e is the most rules light version of the game.
By removing the various subsystems of Old School and consolidating them you empower a player to gain system master easier (which speeds up gameplay) but also it allows a GM to improvise something while remaining within the bounds of challenges the party might encounter.
As a concrete example....let's say I want to include some undead in my adventure design. The party had a 9th level cleric. In am Old School game I would have to break out the turning chart to see what kind of undead the cleric could turn, destroy, or have no power over.
In 5e I don't have a codified chart, as any undead can be turned by any cleric. This frees me up as the GM to include, let's say a mummy for theme purposes" without fear it isn't going to work with the party.
I feel more empowered to improvise in a system that doesn't have those codified bits that limit things.
As another example, let's take the idea of a surprise round. In 5e it's very simple and relies entirely on GM fiat to decide if any person or thing is surprised. Then you run combat as normal and all the surprised beings can't take a tions on the first round. That's it...the entire rule.
I can't even begin to tell you the rule for surprise rounds in 1e, because we never used the actual rules when we played back in the day. I know it's an opposed d6 roll with I think mtiple rounds of surprise, but then you have classes that roll different dice and monsters that have different interactions....and it's kind of a mess.
Finally I'll point to something pretty basic, the weapon table. Weapons have reach values, initiative modifiers, different damage versus different sized foes, and even adjustments based on the AC of the opponent in 1e. 5e has nowhere near this comexity.
Some of those were templates that could be applied to any creature while the others could get a template to shift them thematically & have a challenge that fits where you want it while being called a mummy with whatever flavoring refluff that suits. "No I'm sure I can just do it because any undead can be turned by any cleric nerf me with gm fiat if you don't like it" is a serious negative not an improvement. Players are almost never going to be unhappy to find that something works better like having turning buffed to work on any undead no matter how long the list of cool boons you give... the same is not true of nerfs