That should be a while, the B550 can't be used with older CPUs and APUs and there are still a ton of them in distribution. Some old CPUs will have long service guarantees that spares can be bought for corporate machines.
I was pining the other day for the old AM1 platform to knock up some *really* cheap boxes. But frankly, an A320 board and Athlon 3000G isn't that much more. Unless you buy ITX of course, which these days has a totally mental premium attached to the form factor. I mean, £43 gets you a cheap A320 motherboard
https://www.amazon.co.uk/MSI-A320M-P...dp/B07W838R4F/
Or you can go ITX for £104, that's over twice the price for something not really any better just compressed a bit.
https://www.amazon.co.uk/ASUS-A320I-...dp/B07THL4C87/
As an aside, in is interesting to see that with current street prices that MSI MAX board is 1p more expensive than the non MAX version, despite the full retail price being a few quid more. I wonder, is that because volume is favouring the large ROM board or is the overall market so "meh" about rom sizes they have to discount them to shift stock.