Less character diversity as Feats become less attractive
When you get an ASI, often the most useful and powerful thing any character can do with it is boost their primary ability. Most players will not consider taking a Feat until their primary ability score is maxed at 20. By removing the cap, you make it so this "default-best-option" of raising your primary ability score is always available, and thus make players choosing Feats to make their characters more unique less likely.
Characters are more like glass cannons
In the same vein, now that there's no limit to your primary ability score, raising secondary ability scores for defense is much less attractive. Where before a Wizard with 20 INT has reached the limit and might consider raising DEX or CON to increase their longevity, now the siren song of higher Save DCs and most importantly an extra spell prepared per day will almost certainly cause them to raise INT instead - making them more powerful offensively at the cost of their defense.
This will make it harder for you to create good combats, as you try to find the knife-edge balance between "not any threat" and "kills them". (I have extensive experience with a highly offensively focused, low-defense party, and this measure would make that problem considerably worse)
Prepared spellcasters are more versatile than designed
ValhallaGH has already pointed out how spellcasters will become more powerful with high save DCs. As well as that, the number of spells clerics, druids, and wizards can prepare is tied to their casting stat. The game is designed so the bonus that stat provides to max out at 20 - if it can go higher, those classes are being given more versatility than intended (to the detriment of the niches of other classes, which may become overshadowed).
Lastly, a frame challenge: do your players really feel this way?
When I started playing D&D, I was highly focused on optimisation. I raised my ability scores with ASIs even when there were cool and interesting feats available, because I thought it was more optimal. I chose a less interesting subclass because I thought it was more mechanically powerful. And several levels in I realised I had been making all the wrong choices, because I had not built my character with any interesting mechanics available, and I was now bored with how my character played in combat, even though the numbers I was adding to my rolls were big. Perhaps your players really do only care about higher and higher numbers - but perhaps you/they just think they do.
