If you want developers to be in any way responsible for software bugs, you need them to have an income stream to justify continued development/maintenance.
I'd suggest that in an online connected world, you want to be able to hold software developers accountable for the security of their products and expecting them to maintain software forever based on a one time purchase is not viable.
If this product was used on a device that is not network exposed and no maintenance was expected - sure, one off purchase would be viable. But that's not the world we live in today.
And whilst you may think that a movie production app like Final Cut doesn't matter security wise... its still going to have to process potentially malicious input (e.g., maliciously crafted video or other media file) that could compromise the machine.
Let's take a hypothetical here.
Purchase cost: $500
Subscription cost $50/year
Whether I pay that outright or subscribe for 10 years is an equivalent source of income - assuming neither price adjust for inflation the here and now money is better. Then it's just a matter of managing the instantaneous income with longer term expenditures. If the product is good, new customers will also flow in, not just as you penetrate the market more but also as the previous users retire and new users pop up.
I am also fine with paying one-time fees for feature updates that can then in turn also help fund maintenance across the board. You can even maintain a singular codebase and effectively have the feature releases have their bundle of features behind an IAP.
And on top of that; The subscription, to some customers like me, will in fact be $0 cause I refuse to give myself the stress of managing another subscription service for a program. Now there may be some who'd prefer a smaller upfront but continued fee who would then also be lost if you only offer an outright purchase, both options is probably preferable, but I reckon there are more lost sales for subscription only than buy only.
Now that's the general case. For FCP specifically, the FCP development team does not even need to make money off of FCP specifically. Apple needs to make money off of FCP holistically. There are people who buy Macs and potentially now iPads specifically because of Final Cut. These people will buy several Macs throughout their lives once Final Cut is established as their editor of choice. They'll buy other things on the App Store maybe, potentially get an Apple Music subscription, etc. etc. FCPX on the Mac has had a fairly low price and a lot of updates this whole time, and I am certain it brought a lot more value than its price tag per sale. Same for Logic for that matter. I chose FCPX over Premiere when I initially got into video editing specifically because of the buy vs subscribe pricing model, and it only entrenched me further in the Apple world.
Similarly, Black Magic can offer such an excellent trial and good pricing on DaVinci Resolve because they have a whole brand they build around it. Cameras, colour editing equipment, etc.
But alas, I think I'm losing this one. I'll have to give in and give up when they start charging an oxygen subscription