What he said. Lineage is just Android with new features added and occasionally a minor facelift. There is very little functional difference between stock Lineage and stock Android. Also as he mentioned, Lineage is great to get old phones on newer versions of Android.
As for dual booting, the partitions on newer android systems are rather fixed. You can wipe and install stuff on the partitions, but you can't make new partitions. {EDIT - I was wrong, you can partition phone storage. It's not easy and requires more learnin' than I have, but it's possible.
See petteyg359's post below.} That would be a prerequisite for a dual-boot system. I don't think the hardware manufacturers are keen on letting people mess with partitioning a phone. There is just no reason for them to even bother thinking about it.
I can say with absolute certainty that people wanting to modify their phone are far less than 1% of the market. They have no motivation to write software or consider supporting third parties that may want to write software to do what would be needed. If there's no money in it, they're not going to bother. So it's not a hardware issue. I'm sure our pocket computers are more than capable of running a dual-boot system. We just won't get one because there's nothing in it for them.
Then there is the driver problem. A main reason it's hard to get new Android versions of Lineage to work on older hardware is because the hardware is no longer supported with new drivers. That gets exponentially more important if you want to run a completely different OS (Windows mobile or iOS for instance). With no drivers, even if you had partitions on which to install multiple OS'es, your phone will never boot up.
All of that said, there IS a potential for dual-booting different versions of the same OS version with Lineage, for instance to test a new build. I used to build Lineage so we needed to figure out of the new build borked anything. The way Android updates work now (since Android 8/Oreo) is that they install the OS on one of two hidden OS partition, using the same user data in the data partition so all of your settings/apps/etc don't change. When the system reboots after the update, it boots to the partition with the newer OS version on it. There used to be a way to switch between those partitions. I'm not sure if that's still a thing as it has been two years since I have built anything. This -can't- be done with two different versions of Android or two different OS'es (i.e. Lineage vs. a different non-LOS based OS like ionOS) because the single data partition is not interchangeable unless you want problems.
Finally, the risk of bricking a phone is very, very real. That is usually a soft-brick and recoverable, but there are times when you mod a phone into a useless brick. It's not like people are going to be able to de-solder their storage chips and install a new one. You'd have to replace the motherboard entirely, assuming there aren't hardware versions that make a different motherboard not work with the rest of the phone.
(Also, I'm not sure where you get the idea that manufacturers are handing out keys for rooting and modifying phones. They have increasingly made that harder and harder year after year. You haven't been able to root Snapdragon-based Samsung phones for years. The only phones that I know will intentionally allow you to modify anything are Pixels you buy from Google [and you're giving up any secure applications like banking/Google Pay by opting to do so unless Magisk Hide works again] and OnePlus.)
