I think you've laid your arguments preeeeety clearly, especially on the saving throws not scaling properly. I can't help but wonder if it would be possible to "fix" this somehow.
I do have to note however, that this is going to be a problem for tier 3 and 4 play mostly. At lower level the gap between a good and a bad save is not as immense.
That being said, I have been noting with alarm the proliferation of "things" that increase save DC or give disadvantage or force a reroll etc (magical items like the arcane grimoire, spells like silvery barb, or (sub)class abilities like a divination wizard).
Tonight we had a fight vs a manticore, a level 4 party (1 wiz, 1 apothecary (think the hulk), one ranger, one rogue and one cleric). We manage to lure it in a bad position for it, and the wizard zapped it with a phantasmal force that completely messed it up. Sure, it took the rest of the party to clobber it - and 4 level 4 PCs can do a lot of damage! - but the spell made the fight a joke. And because the spell targets int saves? ooof