Every six years is enough for apple and other companies who have other sources of revenue and have staked out this high quality niche. But androids, as an example, are more of an average 3 year lifespan if I'm not mistaken, which is closer to what Synology would probably want to achieve but cannot.
The comparison to phones is shaky here. Phones bring substantial performance and feature improvements over 6 years, HW and SW. Synology on the other hand still uses a 5-6 year old CPU and 1Gbps connectivity in their home "plus" line. The OS development is mostly security updates with substantial feature releases few and far between. I expect this from a NAS but it's not at all comparable to a phone.
Forcing their drives is a tax on top of an already existing tax. Synology already charges a premium for lower end specs than the competition. If that's not enough to compensate for the longer upgrade cycles, and they want a hand in every cookie jar it's just going to be a hard pass for me.
I upgraded my Synology box every few years and this is exactly the time I was looking to go to the next model. And I'd pull the trigger and buy a current model before they implement the policy but the problem is now I don't trust that they won't retroactively issue an update that cripples existing models somehow. QNAP or the many alternative HW manufacturers that support an arbitrary OS are starting to be that much more attractive.
I don't think mobile is the right comparison. Those ecosystems are explicitly operating on the assumption that they will profit through the software ecosystem (app store revenue).
Synology seems to have gone entirely the other direction here. Most of their software is given away for free, but the hardware is being monetized.
Additionally - the hardware has different operating constraints. I think the big deal for Synology is that they probably assumed that storage need growth would equate to sales growth.
EX - Synology may have assumed that if I need to store 1TB in 2010, and 5TB in 2015, that would equate to me buying additional NAS hardware.
But often, HDD size increases mean that I can keep the same number of bays and just bump drive size.
Which... is great for me as a user, but bad for Synology (this almost single handedly explains this move, as an aside - I just think it's a bad play).
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I'd rather they just charged for the software products they're blowing all their money on, or directly tie upgrades to the software products to upgrading hardware.
Sounds like this is the problem with Synology... How are they going to make money when their products are so good!