Double edged sword...
On the one hand, not deploying the update to older phones would mean that they remain more functional..
On the other hand, by not allowing handsets to get the latest software, unless you're releasing patches for the older software as well, invariably, the older devices will be vulnerable to hacking.
In my mind, a phone should not be designed, either by hardware or software limitation, to only last 2 years (batteries excluded - I'd class batteries as a consumable).
I have a Galaxy Nexus - and Google have dumped support for it (allegedly initially because TI stopped supporting the mobile [TI OMAP] chipset within), but thankfully Cyanogenmod have been releasing at least major security updates for it. Sure, it's slow, and sure I want a new phone, but should I *need* a new phone? If I change the battery, the thing could remain functional until either the apps grow too big to be able to run (geez, how much resources does an IM program need, I'm looking at you, Facebook Messenger!), they finally stop doing security updates (connecting unfiltered to the wider world with a machine that no longer receives patches is asking for something to happen), I physically break the phone, or the flash wears out. My previous phones I've also ditched because they either couldn't do what I needed (2280, N70, N95), were buggy (Hyundai HGC-310e, N95), or their network support was killed (CDMA - HGC-310e, 2280).
If manufacturers are worried about a revenue stream, then perhaps they need to think about a licensing model around software updates to fund things. Abandoning the security of working devices because you want to sell some new ones, when they're still perfectly functional for browsing theweb, sending and receiving emails, calls and texts etc. seems like a massive waste of resources... especially things like rare earths and other minerals that often come from all sorts of sensitive areas in the world and can't easily be recycled.
The same applies to tablets - Android landfill especially - I like Android, but some cheap and cheerful Chinese OEM isn't going to give two flying .. well you get the idea ... about providing after sales updates for some Shenzen special, or open it up to easily allow third party updates when they could just sell newer devices.
Networked devices, be they phones, tablets or IoT, unless they're firewalled off from the world with very specific controls, they need to be updated, and updated and updated. Printers used to be the "forgotten networked target" on networks, now it could be a lightbulb!