1. Only if you bought a super duper expensive and popular device which is guaranteed to have a large community behind it. There's still one guy who has been making custom ROMs for my Android tablet for the past six years, but not everyone is that lucky. Even then, you put a whole lot of trust in some random nobody developer on xda-forums ("hello guys I made a new ROM, check it out, right now it doesn't boot though, I also won't show you the source code or the changes I did"). That said I don't think this is that good of a point anyway if you look at how long it takes for Sailfish updates to arrive on the Aqua Fish, but maybe that just has to do with Intex being a terrible company.
2. I'd rather not have to install patches just so my device has a smaller chance of spying on me.
3. Same as 1, you can only get root access to super popular devices because no one bothers with the rest. The only way to get root access is by looking for and abusing a security exploit. Wonderful!
4. Same as 3.
5. Same as 3.
6. Google is an advertising company, you are the product.
7. The only Android applications I've heard of that don't work properly with Alien Dalvik are those that have a hard dependency on the malware Google Play Services, which also don't work on any Android system without Google's proprietary malware. What a coincidence! Besides, the point is that you can run Sailfish, regular GNU/Linux and possibly Android applications, instead of exclusively Android applications and the occasional GNU/Linux application someone bothered to port. Which you can also only use if your device is rooted.

Having to exploit security loopholes to be able to do anything at all... Maybe it floats your boat.