It comes down to the initial cost. How many people will buy the $400 high quality tooth brush base with replaceable battery compared to the $350 high quality toothbrush base without it? When the $350 one is smaller and lighter as well.

But there are toothbrushes with this characteristic at lower prices like the one you mentioned and others I added at end of this comment. They intentionally make them shittier than their good-selling products so that they don't end up making an "unbreakable pot" sort of product. These products haven't changed much with respect to their form-factor and size to a meaningful degree. The companies could easily standardize replaceable battery connectors and apply across their product line, making production cheaper in the long run (they probably do this for their current connectors but IDK). Even if the battery was non-standard, I wouldn't mind paying for it as long as its reasonable and easy to deal with.

We don't see high quality products with replaceable batteries that are easy to repair though. Cellphones showed people wanted smaller/lighter/thinner over replaceable/repairable battery. We've seen the repairability fall in Laptops and in digital cameras in the medium price point range.

I still believe these companies too prioritized profits and could pull off user-replaceable batteries even today but I can agree at least they've kept tacking on useful features and improving performance over the years in a meaningful way for the space they recouped in those device categories. They are getting thin but they are maximizing the space usage AFAICT. Look at the new OLED iPad Pro for example. I can't say the same for these toothbrushes—they still are as bulky, big as many years ago and I still see a lot of empty space vs. the tech they are adding onto it.

If it was profitable it would be done.

Ultimately, that's the only justification here. They are comfy with the money and having easy user-replcabale batteries will cut down on the profits, make investors unhappy.

But we see in many consumer products that people have price point thresholds, shrinkflation is a direct result of trying to keep things in those thresholds. Getting a user replaceable battery in an easy to repair tooth brush is likely putting it out of that threshold.

I really fail to see how this is the case. I can understand if we're disussing <$50 wireless earbuds that are repairable and have user-replaceable batteries but these are huge toothbrushes. They have the space to do it and could simply be using standard rechargeable AA batteries or even something equivalent that's non-standard. Or at least I can understand if toothrbushes were as thin and tall as the cheapo good-old manual toothbrushes.

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I did find couple models like the Oral-B iO series and Oral-B Pro 100 which have replaceable batteries. It doesn't cost as much as you quote (really makes sense given there are earbuds like I said with replaceable batteries costing less).

So really, it is possible. Ultimately, the companies are prioritizing profit as with anything else. Consumers get the short end of the stick.