The current designs allow for fitting more battery into a smaller space because the available space can be non-contiguous and can contain odd angles while a replaceable battery comes in whatever shape you want as long as the shape is rectangular. By not having accessible batteries the vendor can seal the phone much better and increase its water-resistance. Your older phones were larger, heavier, not water-resistant, and had one more set of user-accessible failure points.

I can’t remember a single phone with a replaceable battery that did not need tape to fix it after a year of usage... and I never bought a replacement battery.

Your experience is either imaginary or ridiculously atypical. Its literally not a real thing. Devices with replaceable batteries don't normally fall apart unless you are in the habit of dropping them all the time. This isn't a design consideration or a justification.

Phones are designed without user replaceable batteries because it makes it slightly cheaper to manufacture, generates revenue from users who require replacement, and makes it less desirable to keep an existing device vs buying a new one.

My Nokia 6230i (which I still sometimes use as a backup), Nexus S, Xperia Pro, and LG F3Q would disagree with that assessment. Maybe treat your devices a bit better?

Well if you personally didn't have a positive experience with such a phone, then I guess it's axiomatically impossible!