Now that Motorola is wholly owned by Lenovo, do you think they are going to keep up the resources to update old phones?
Even if the phones are pure Google for all intents and purposes, Lenovo would still have to devote time and money to keeping cheap, low margin handset hardware up to date when they see no benefit from doing that. It's ok for Google to do that, they see money from the play store and it benefits them to have pure Google phones with recent OS updates on them. Lenovo only makes money from hardware so it benefits them to sell you a new phone rather than update an old one.
I know I am probably too cynical, but do you think Motorola phones like the G and E will be updated to android 5 and beyond?