It's ridiculous that you cannot open the previous version of a Pages file in the current version of Pages! What an absolute joke.
Did OP delete those old versions? I don't see what the problem is here.
He got a new machine.
I would not mind buying a floppy drive and all necessary adapters, install Atari ST emulator and write Kamenicky encoding converter in Python to convert 1st World Plus documents from '89 if I had to convert documents of my parents. I would not be complaining about obsolescence of the format at all.
My problem is not technical, it is about user experience and productivity.
1994 -- switch from Motorola to PowerPC.
2006 -- switch from PowerPC to Intel
Can I build a flappy bird clone? Sure. Should I? Hell no. I have more profitable projects to work on given a fixed amount of time and resources.
If you think this is money left on the table, then build the 'small utility' to convert the format yourself. If you make money, huzzah, you were right. If you don't, then you were wrong, and this is a niche use case that isn't worth the investment.
That would violate the Keynote license agreement. So, we're back to needing Apple to help out here.
edit: from Section 2A, Permitted License Uses and Restrictions... i) to download, install, use and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running OS X (“Mac Computer”) that you own or control; and (ii) if you are a commercial enterprise or educational institution, to download, install, use and run one (1) copy of the Apple Software for use either: (a) by a single individual on each of the Mac Computer(s) that you own or control, or (b) by multiple individuals on a single shared Mac Computer that you own or control. For example, a single employee may use the Apple Software on both the employee’s desktop Mac Computer and laptop Mac Computer, or multiple students may serially use the Apple Software on a single Mac Computer located at a resource center or library.
Doesn't that cover the suggested use?
Except, right now, it doesn't. If that was their primary design goal, than it has not been achieved, because their web version still lacks features that are front and center on the desktop. So, you can't use that as an excuse.
The great thing about #1 is that if Apple really did leave money on the table here, any third-party developer can collect it. Just put a Mac with two versions of keynote behind a REST endpoint and charge a buck or two per conversion. If you can turn a profit at that, not only do you get the smug satisfaction of clearly winning an internet argument, but also there's a cash prize.
If, however, that sounds like a waste of a perfectly good weekend, an economic bet that is unlikely to pay off, it would be an equal waste of a weekend for an Apple engineer. Apple is not a charity; they are a business that takes calculated risks, and they didn't like this one. Did they miscalculate? If you think so, there is no reason not to fill the gap yourself.