Do have some names of the software that is indispensable on the PPC Mac? I would love to know.
The most indispensible to me – as an amateur photographer with a vast collecton of slides – is the software (Nikon Scan 4) to run a slide scanner of the same era, which still works fine. In theory, I could reverse-engineer drivers for the scanner, but life is short. There's also Vuescan and Silverlight, but why shell out hundreds of dollars when I already have something that works just as well? I _ought_ to buy one of those, but the path of least resistance is to just use what I already have.
The other software is also maybe more convenient than absolutely indispensible. DNA Strider, for example, is great at what it does, but at this point if I had a little bit of extra time I could write my own clone. It's not something I use very often anymore. But on rare occasions I still do. Oddly, given how many molecular biologists there are out there, nothing quite as user-friendly emerged after the Mac's transition to Intel.
And then of course there are multiple big proprietary apps that switched to a subscription model shortly before or shortly after the transition, especially for domain-specific stuff. Not absolutely indispensible, but there's no way I could afford to spend thousands of dollars a year on software subscriptions. Especially since it's not all that often anymore that I need to use them. There are free (as in freedom) alternatives for most of them, but there's some friction involved in switching to the free alternatives. For example, I use the last PowerPC-compatible version of MATLAB (which has in the meantime switched back to offering a perpetual license again – at quite a price!), but recommend GNU Octave to those friends who balk at MATLAB's prices. And everything I've done in MATLAB it's now possible, with some effort, to do in Python. And Python on Apple Silicon is probably significantly faster than MATLAB on PPC!
For decades, macOS has been only a tiny slice of a market that's dominated by Microsoft Windows. I think currently, maybe 90% of the desktop/laptop market is Windows, 9% macOS, and 1% the various Linuxes. Broadly speaking, the breakdown of effort by software companies is roughly the same. So yes, I didn't mean _absolutely_ indispensible – Anything is possible on a Turing-complete machine! I meant relatively indispensible given time and money constraints.