I generally agree. We've always taken the "grand-fathering" approach, but it can get a bit more complicated as your product and offerings grow. If someone was paying $XX /mo for your initial offering, and a few years later, you're able to provide a number of new features within that product, I don't think you necessarily need to continue providing those extra features simply because someone signed up earlier.
To your point, you may face a backlash. But it also costs a lot of time and $$ to support new development and features. Therefore, having some backlash may still be the right decision in the medium/long term. Can be hard to weather that storm initially, though.
The features that they paid for in the first year have already been paid for by the customers. While it costs the vendor time and money for new features it doesn't cost them money for old features. Therefore under a subscription model that payment should be covering the cost of new features.
I think I'd reply to this with a "yes and no" kinda thing. For us, maintaining anything requires time and $$. This is, in part, because technology changes. Even in a vacuum with no new active development, money and $$ is required to ensure everything stays up and running.
eg. as our use base has grown, so too has our database. And so while the optics of it from the end-user POV is that the product has remain unchanged, we've had to invest in database and scaling services in order to continue to meet the initial offerings.
It's not as simple as that, but you probably get the gist of it.
A more relevant example might be a web service that emails you once a week. If the mailing service that powers this requires DNS or integration changes to help fight spam, over time that will require time (and perhaps $$).
If I'm misunderstanding your point, let me know :)
To your point, you may face a backlash. But it also costs a lot of time and $$ to support new development and features. Therefore, having some backlash may still be the right decision in the medium/long term. Can be hard to weather that storm initially, though.