That works fine for purely client-side software, such as was common in the 90s. Photoshop would be a great model of that. For such software, the only, or main, cost is R&D, and you pay for their R&D for each new version.
It doesn't work so well for software like this, where one of the main selling points is that all your notes are stored in the cloud and can be retrieved on any new device (or by opening the website in a tab). That's a marginal cost which is directly proportional to your actual use of the tool itself - not an R&D cost which relates to developing the code.
I understand objecting to software where the SaaS/cloud aspect is tacked on as an afterthought, to justify a subscription model – again Photoshop is an example – but that's not what's going on here. It's eminently reasonable for this software to be subscription-based.
In short: It's reasonable for a car to be a one-off purchase, but it doesn't work the same way for the gas.