Even if it's only 1% that's still $6 million bucks. I suspect there are lots of people like me who have the technical ability to flash a custom ROM but just don't want to spend the time and effort on it. Reading through a 200+ page forum thread and dealing with the rough edges of these custom ROMS requires an investment of time. There are also all the business/enterprise IT departments who aren't going to touch a random custom ROM they found on the Internet. They need some official upgrade path. Most companies don't want to buy a new fleet of phones very 2 years.

Most companies have no problem buying a new fleet of phones every two years. Especially when the price is in the general area of office consumables.

Just the battery in the phone will last barely two years and you'll spare yourself a lot of troubles by buying new phones.

If it ever came to that I'd probably just switch to a Nexus or an iPhone. If I buy a device with the expectation that it will be supported for a period of time, I don't expect to pay additional for the updates that were originally promised.