Well, I don’t know how much complexity would be involved since I don’t know how the code works. But obviously something is being done to tell the TB to stop charging at 100%. So I can see how it COULD be fairly simple to have it check for two numbers - 100% and 80% and certainly shouldn’t be a big deal to add a switch in the app.
As for the usefulness and how often you want to charge, I figure for a lot of people, as long as it last for an entire day, regardless of how much you are using it, they will be satisfied with charging each night.
Now, many people will have it last much longer because of less use. So, suppose you have someone who goes a week between charges. They may get used to the idea of only charging on a specific night each week which becomes an easy to remember habit. If limiting (though this would only be an option) the charge to 80%, can they still make it a week? If so, then this is still a nice option.
The only negative I can see are people who have an easy to remember routine now, but the reduced charge would put them into an odd charging cycle which is harder to remember, thus requiring them to pay more attention to the remaining charge. Like if you go from weekly to every 5 days.
The important issue is how much longer would the total battery life be with the reduced charge? If you get four years now before a heavy user wouldn’t get a full day, but with this change that might take 6 years. That would be a nice benefit.
UNLESS there are other things which a heavy user would wear out by 4 years anyway. Of course, people may opt for newer models just as they replace good phones with newer phones, but I suspect they would either give the old TB to someone or keep the old one as a backup. Or as one to keep in their travel kit so they don’t have to worry about forgetting to take their main one.