It depends on your definition of dismal. Any phone repair shop can open phones and replace batteries. There are also disassembly videos for most models on YouTube. I looked at them, ordered parts and replaced the battery on my Samsung A40 when it started to discharge too quickly. I also replaced the camera after a hard crash on the floor broke the autofocus. Maybe not having Samsung shops makes all of that dismal but I actually prefer to have many small independent shops around the country.
Market fragmentation among Androids means that nobody stocks parts for anything and the margins are so low, a lot of shops will only do the brands they want to.
I couldn't find anyone to replace the battery in my Google Nexus phones, every time I tried, in a major metropolitan area. Ended up having to do it myself.
You can walk into nearly any cell phone shop in the world with an iPhone and walk out with a replaced battery because the staff know them and they stock parts for them.
I'm not sure that the problem is fragmentation inside an operating system. It's how widespread some brands are. I never had such problems with any Samsung phone I owned, even a Sony Xperia one years ago. I guess that it means that Apple, Samsung and even Sony (back at the time) are more mainstream than Google and for shops it's not worth to keep spare parts of Google phones.
To be fair, the last time I had to replace a screen of a Samsung because it dropped down flat on the screen instead of on a corner, I had to wait one day for the replacement part to arrive to the shop. I put the SIM in the old phone or in my tablet and went back the next day. I'm not replacing the screen myself, it looks to require some extra skills compared to a battery.