While I agree that's been the case in the past, supported devices are fast enough now that it really doesn't slow them down anymore. Even the lowest supported device runs well on iOS 10, and it has gotten official updates years longer than all android devices I know of.
Sure, one could install an updated and more secure rom (assuming it exists for your device). But, the vast majority of users don't care or won't bother to go through that process, rendering it a completely ineffective solution for the general consumer market.
