I'm seeing two different arguments here:
"High level characters shouldn't fail important saves all the time"
vs. "Characters should get better at making saves as they go up levels"
which are related, but hardly the same argument.
Pretty much. I wouldn't mind an opt-in system if players have an actual effective method of increasing poor saves. It doesn't exist.
If the PC starts with a pair of 14's in their poor save stats -- 14s not 8s I am not talking abut 'dumping' poor save stats -- at level 9 the character will succeed a poor save 30% of the time. This falls further as the character levels until it hits 20% once his opponents get +6 proficiency.
The best -- absolute best -- a PC can do is spend maximise his stat bumps to increase his save attributes. Moving a 14 to 20 takes three stat bumps. Each bump grants a 5% improvement. A PC has 2 poor save attributes. So spending six bumps means his chance of actually saving against a poor save has gone from 20% to 35%. At 9th level, a PC cannot hit 50%. At high level, there is nothing a PC can do to get a poor save over 35%. I-in-5 to 1-in-3 is an improvement, but it does not move the needle from "will occasionally save" to "can reasonably hope to save".
If we assume point buy, provided array, or less than incredible luck from rolling, assigning two 14s to your weak saves is already a hefty price in negating a slew of character options. The array is 15, 14, 13, 12, 10, 8. Racial mods can add up to 3 (if not human) so you can put the 15 into your main stat (augmented to 17). So let us make the 13 a 14 and the 15 a 17. 17, 14, 14, 12, 10, 8. 17 into your primary stat, 14s into your poor saves, 12 into your good save (if not a Rogue), and 10, 8 into Int/Cha.
Let's assume a Fighter because it is the best case since they get the most stat bumps. (Mountain Dwarf works; his Con ends up 13 instead of 12).
The Fighter's good save now suffers over the lifetime of the character. By 9th level, the Fighter who manages to get his Wis save to the heady heights of 45%, will see his Con save start at 50% and fall to 40%. The 9th level Fighter makes no saving throw half the time.
His saves are Con 40%, Wis 45%, Dex 30% at 9th level. He can't improve Wis further.
So the Fighter get six points to improve Str, Con, or Dex over the remaining levels.
Without further improvement by level 17, his saves will be Wis 35%, Dex 20%, Con 40%. If all bumps go into saves, the PC gains 15% between Dex and Con.