well it's multi-layered...even philosophical, I suppose
Not so much about an imediate need for >20 continuous hours
1) yes concern about when the battery ages, 20 hours becomes 19, then 18 and it's borderline...then 17 and it's a failure. It's just too close to actual need without a proper safety factor. I generally wear my aids roughly around 15-16 hours per day, but around 18 hours per day sometimes happens
2) and also, without the safety factor of extra time there are a lot of very plausible scenarios that can cause a serious inconvenience. An inconvenience to me because I can get by without my aids, but for some folks it's a real problem potentially. There's a current thread about this whole topic so I'll try not to belabor it here too much...but what happens when that overnight charge that you're counting on fails for whatever reason, and you wake to find dead or nearly dead hearing aids? Do you have 1 hour that you can sacrifice to charge them up to around 40%..... and then will that 40% get you through the rest of the day?
Summing it up, I'll not happily accept rechargeable hearing aids until three things happen
a) They give at least two full days of use on a charge...that means in the ballpark of 30-35 hours at least. I reckon that should be enough for most folks to sort out a reasonable charging problem.
b) The rechargeable aids can charge without the need for a proprietary charger device. Think cellphones and other devices now days...can be charged with any USB-C cable and many with wireless charging....
c) the rechargeable aids can charge FAST...and I mean FAST. Like my now 5-1/2 year old cell phone that claims to charge to 50% in 15 minutes and 100% in about 30 minutes. It might seem ridiculous at first, but there have been several times when I've woke to find that I hadn't plugged it in properly (or forgot to), and I can get a useable charge to get me through the day in the short time it takes me to shower. The reality with a cell phone is that it could be slower than this becasue I can plug the phone in while in teh car driving, or plug it into a battery bank in my pocket and it can continue to charge while I go about my business...but a hearing aid kinda needs to charge that fast becasue it's not realistic to think that you can charge it and use it at the same time