Sage are fairly determined to get people onto a subscription model for their software, because they like the "pay and keep paying" model, over the one preferred by people like me, which is to pay once and then just keep using it until it actually *needs* to be upgraded. And you can still get Sage's main products on a perpetual licence - but for one thing.
For day to day work accounts and bookkeeping work, tax law changes are irrelevant - it's down to the person using the software. There is one change coming RSN, however, that does force an upgrade: Making Tax Digital.
If you're using a Sage product that you have on a perpetual licence, unless Sage have changed their minds since I last commented on this, if you want to be able to submit VAT returns under MTD, you have to *subscribe* to the MTD module.
HMRC don't charge for access to the API.
The API, once finalised (and I believe it now is), shouldn't be a constantly changing, moving target. It's possible that it might change occasionally, but as a general rule it should remain fixed.
Therefore, in my opinion, there is no justification for a subscription model for the module. Sure, they may update and improve the software over time, but if people want a version with a better bloody UI or whatever, they can buy an upgrade - and similar if and when the API does change.
But to insist on only selling it on a subscription model is, in effect, to tax businesses for being able to pay a tax.