The problem with speed and durability isn't so much ext4 as sd per se. You didn't notice it the same way with the old way of moving apps to sd because that only moved part of the app (apk and library components) and didn't move the app's internal data. Hence sd speed was only an issue with loading apps rather than running, and there was a lot less writing than there is to the data area so less risk of card failure (sd cards can fail anyway - I've had a lightly-used fat32 format Samsung card crap out terminally after only a few months). I used to use ext4 partitions for apps back in the bad old days of devices with 150MB of internal storage, but never moved app data to them. You can still do that type of thing if you want, it requires root but it also did back then.
Yes, some manufacturers choose not to allow adoptive storage. That's the manufacturer's choice. If it's important to you, don't buy from manufacturers who don't support it. Personally I agree both with you and with the manufacturers who don't include it: it's a bit crap, and I would not use it myself.
The not allowing writing to sd by default is a security thing: fat32 doesn't support file permissions under Linux, so anything there could be altered by any other app. Google over-reacted to this in 4.4, where they basically didn't allow user apps to write to sd. That's since been rectified, in that an app can ask for permission and you can choose whether to grant it or not, but amazingly 3 years later many apps have still not added this option. It's been there for long enough though, so complaints about an app not being able to write to SD should be addressed to the app developer (not defending Google, they make plenty of bad decisions IMO, just saying that app developers have the choice here and so it's their fault if they've not provided the option).