Samsung have always updated their flagships with two major OS upgrades as well as three years of monthly security updates. Last year, Samsung added an extra year of quarterly updates for the Galaxy S7 and S7 edge, and are likely to do the same with the S8 soon. However, if Samsung can do this, they can release three major OS upgrades instead of two. Here’s why.
Reason #1: Devices that miss out on updates are perfectly capable of running them

Last year, the Galaxy S7 did not get Android 9 Pie, despite being perfectly capable to run it hardware-wise. The S7 and S7 edge ran fast and smooth on Android 8 Oreo, yet Samsung did not support them on Android 9 Pie.
It has also been proven the S7 and S7 edge can handle Android 9 Pie and even Android 10 thanks to custom ROMs. Several ROMs of Android 9 Pie have been released for the Exynos variants of the S7 and S7 edge, including a port of the Note7 FE (which started off on Android 7 Nougat) Android Pie release, which runs great on it.
There was no reason for Samsung not releasing an update other than that the phone had received its two major OS upgrades and therefore was not eligible for a third.
Reason #2: Phones cost money

In 2016 when the S7 and S7 edge were released, with the S7 costing £569 and the S7 edge costing £639. Half a year later, the iPhone 7 and 7+ were released and they cost around the same depending on what variant you buy.
However, the Galaxy S7 and S7 edge stopped getting major OS upgrades in 2018 when OneUI based on Android 9 Pie did not support the devices.
Meanwhile, the iPhone 7 and 7+, released half a year later, are still getting major OS upgrades and are expected to get iOS 14 when it is announced this year.
The Galaxy Z Flip costs £1300 but is likely to be subject to the same terms. So in four years time from now, your Galaxy Z Flip will likely be obsolete and have only received two major OS updates. As I wrote in a previous post, this expensive phone deserves more than two major OS upgrades.
Reason #3: Even other Android manufacturers are doing it now

Last year, Google surprised us and released Android 10 to the original Pixel and Pixel XL. These phones were released in 2016 running Android 7 Nougat and have received updates to Android 8, 9, and 10. The S7 and S7 edge were launched half a year earlier on Android 6 Marshmallow and only got two major OS upgrades to Android 7 and Android 8.
Of course, Google own the Pixel brand as well as Android and therefore can easily make adaptions. Not to mention Google updated the original Pixel and Pixel XL phones with Project Treble allowing updates and ROMs to be developed more easily. But this proves how Google are willing to work hard to give their phones a good lifetime of support.
Google are not alone. Last year, OnePlus released Android 9 Pie to their OnePlus 3, a phone that started off on Android 6 Marshmallow. This year, they’re releasing Android 10 to their OnePlus 5 phone which started off on Android 7 Nougat.
Other Android manufacturers can do it, so why can’t Samsung? Nothing except an easily changeable update policy is stopping them. It’s not like all the manufacturers providing three major OS upgrades to their phones are using stock Android either. OnePlus have their own variant of Android, OxygenOS, and they still manage. There’s nothing stopping Samsung from following OnePlus.
Conclusion

To conclude I think Samsung should release at least three major OS upgrades to their flagships because their devices are capable of running a third OS upgrade, and also their phones cost too much to be worth buying instead of an iPhone when Samsung phones only get two major OS upgrades while iPhones get many more. Also, other Android manufacturers have done it including Google and OnePlus, and OnePlus has their own Android variant like Samsung so Samsung should be able to do it as well. The only thing stopping Samsung now is the update policy which they can and should change.