One benefit of subscription over upgrade pricing is that apps are continually updated and gradually getting better, as opposed to developers bug fixing the current version while hoarding new features until a new upgrade they can charge for. I would much rather a great new feature be available to me once the developer is ready to offer it, instead of the developer keeping it hostage for marketing purposes.
Essentially I agree with Anselmo: considering the need for frequent updates.
Some applications require constant attention (like protectors against malware). Other apps are provided as the free access to paid services (like cloud storage). I can see the point, and I'm willing to pay. (I view Microsoft Office 365 that way.)
There's also software I'd like to use, but where my use is too irregular. Like my use of the functionality in the Creative Cloud of Adobe.
There's also software whose makers seem to want to want to keep cashing big on one good idea from the past in order to go beach-combing. Well, not from /my/ money.
By contrast, I notice people like Christian Ghisler (Total Commander) and Irfan Skiljan (IrfanView) who have been devoted to their brain childs for "ages". Christian for a moderate one-time fee, Irfan without even mentioning donations anymore (if I remember correctly). And the bulk of the Linux community.
I'm also aware of the fact that "being a professional" seems to imply that you can be charged business rates for the tools (like software development kits) that you (mainly) use for your hobby project, and I too would try to pass that burden on to those who seem to benefit from my expenses.
Ultimately, I may end up siding with the developers who charge a one-off fee for a given set of features, and then again to unlock future features if you should want them.
As a small business owner, I like the subscription model on software. Especially on professional software like Microsoft Office and Adobe Creative Suite. I can add new accounts on a monthly or quarterly basis as my team grows and reduce after a project is complete and I have less staff. And instead of buying a version out-right, with a sibsctipiotn model, when I activate, I can access the latest and most compatible version.
Even for consumer mobile apps, I support subscription model for some apps. I have a subscription to JibJab for example that I activate during the holidays to send out custom digital cards. I deactivate after the holidays. With the subscription, every time I reactivate, I get access to the lates content.
In short, I think that the presumption from many people is that subscription model is disadvantageous to consumers and is cost-prohibitive or hostile. That analysis ignores the option of activating and de-activating software as needed.
The answer to the question about subscription model is: It depends...
In my opinion subscription is totally acceptable for services where developers has to keep app/services updated frequently like a VPN services, video/audio streaming, surveilance, etc.
I agree that a simple app like a game or a password wallet is not suitable for subscription model.
My App use has dwindled to a point where I only consider a new App if it's going to do something seriously better than the stock/free apps that are available, and even then I don't tend to pull the trigger. After Music/Tv Streaming, cloud services and cameras, I'm at a place I already consider expensive. Akin to the bursting car manufacturers of the early 20th century, there will come a time when the market will dwindle, if relying on this model, and only a few developers may be left with sustainable businesses.
The pay to upgrade model that you said is not available is available. Its how I keep buying each new major release of Tweetbot (my favourite app I've ever purchased). The only non-Apple subscription I pay for is dropbox. I'm never going to subscribe for an app, but I may subscribe for a service (like cloud storage or streaming music/video). Monthly fee for 1password? Give me a break. iCloud keychain is free and better. I bought it pre-iCloud as a one time fee, but that version doesn't anymore and the monthly fee is now higher than the "lifetime" purchase of the app a few years ago.
The underlying business is a key point.
Vertical segments (possibly niche ones) needs subscriptions to survive and scale