Claiming that it must be a service because it is sold as a subscription and not with one-off payment is spurious.

Plenty of software products are sold with both models... for example MS Word and Excel can be bought using either a single payment or a subscription. Using the subscription model does not alter the product which is exactly the same product you get if you buy a perpetual license. (Although buying the subscription does entitle you to ongoing upgrades... it is also possible to get those with a perpetual license + an annual support agreement.)