It would be disastrous for the Dapp.
To keep users on your site you need to reward them. It doesn't matter how good the Dapp is but if there is no upvotes the user base that visited that particular site would leave and find another one.
Loyalty has nothing to do with it either. You mentioned Dtube as an example and what would happen is it would probably lose it's delegation and shut down. It's a two way street and Dapps make their rewards from curating upvotes.
Imagine if musing stopped voting tomorrow. I would miss it for sure but they would miss the money they are making more. These Dapps cost money and time to create and need the user base to claim their investment back and finally make profit.
The up vote is one of the ways the Dapp owners advertise and attract users to their Dapp. Everyone is looking for a way to gst some real support from the blockchain and when a dapp promises such support, everyone would start rushing to such dapp.
Regardless of how awesome a dapp is, it's the support the dapp promises to its adopters that would make most people go and see what the dapp is about. A dapp that doesn't promise its adopters any support will have a very slim user base. Unless the dapp can do what the users can't do elsewhere.
So the immediate effect of a dapp that doesn't support its users with upvotes mostly during its initial stage would be low patronage or a very slim user base