I don;t have too much of a dog in this fight, mainly cos levels above 10 I never see. But I am looking forward to the official line on this, why they chose this way. I just hope that WotC don't just ignore it. In the non scientific 'longer thread means most meaningful to the player base' this issue is right up there!
Most of the damage is done by level 9. By 9th level, proficiency bonus goes to +4 so the effect is already growing noticeably -- it's just not as bad as it will get by 17th level.
Let's assume you your playing a Fighter (because they get the most attribute bumps), that only 3 stats matter for saving throws despite (going by what the rules do as opposed to what the text says), and you start with 16, 14, 14, 14, 10, 10 (better than the base point buy + Human ).
STR 16, DEX 14, CON 14, WIS 14, INT 10, CHA 10.
At level 1 Saving throw DCs are typically 13 and your saves are Con +4 (60%), Dex +2 (50%), Wis +2 (50%) (total 160%)
At level 4 you bump Con to 16. Now your typical saving throw DCs rise to 14 (opponents bump offense) and your saves are Con + 5 (60%), Dex + 2 (45%), Wis + 2 (45%) (total 150%)
At level 6, proficiency rises to +3. You bump Con to 18. Now your typical saving throw rises to 16 (opponents bump offense again) and your saves are Con +7 (60%), Dex +2 (35%), Wis +2 (35%) (total 130%)
At level 8, You decide your secondary saving throws are too weak so you bump Wis to 16. Your opponents have hit their cap for their offensive stat so your target saving throw stays 16. Your saves are Con +7 (60%), Dex + 2 (35%), Wis + 3 (40%) (total 135%).
At level 9, proficiency jumps another point to +4. Target saving throws increase to 17. Your saves are Con +9 (60%), Dex + 2 (30%), Wis + 3 (35%) (total 125%)
Every last bump has gone into defensive stats. Your Str is still 16 and thus you are struggling in combat -- missing out on +2 to hit and damage. Your saving throws have gone from middling / OK to worse across the board with Dex now terrible and Wis only very bad.
The good news is further increases will increase your target saving throws only by changes in proficiency. The bad news is you will slip even further behind even if you exclusively increase your defensive stats.
By level 17, target saving throws hit DC 19. Let's put the last 2 stat increases into Dex so it keeps up with Wis. Your saves are Con +11 (60%), Dex +4 (30%), Wis +4 (30%) (total 120%).
A PC can't get better than that, defensively. A player is unlikely to put every stat bump in defensive stats so take 10% out of the saves to give the PC a 20 Str reducing your total saves to 110%. If you have roleplaying or other reasons to want to increase Int or Cha, take 5- 20% more out of saves for those stat increases.
Lets' review:
- the character's stats were better that those promoted for play.
- The class chosen was a Fighter because it gets the most stat increases.
- Every stat increase went into saves. No stat increase were used to augment offense or to otherwise improve the character.
The result:
It started with middling save across the board and managed to keep up in its one good save and cut its other two saves almost in half by level 17. Almost all the damage was felt by level 9.