Back to the OP:
I‘d wager most LHBS who stock liquid yeast maintain the cold chain right throughout the transport and process. The reputable places I’ve spoken to import their yeast in refrigerated containers. I don’t think a lot of places put the same care into their handling of dry yeast, which will affect the quality.
When I’ve had the same beers side by side fermented with USO5 vs 1056 there’s bugger all difference IMHO when both yeasts have been handled appropriately.
The other thing to consider to make your liquid yeast go a little further (instead of washing or splitting you starter) is splitting the pack prior to making a starter. It’s pretty straightforward. There’s a
thread by Tony which gives a good run down on the process. That way you can make 3 beers out of pure, first generation yeast. You then just build up a starter for each of the vials as you usually would (You could split it further if you really want, but I never bothered).
I did this for years when I was brewing more regularly (currently on a hiatus), and I never had a failure/infection/flavour alteration compared to the original pitch. The longest I kept a vial in the fridge was 4 months.
Probably not as cheap as continually washing and reusing, but it is reliable, repeatable and your splitting your yeast prior to any fermentation occurs.
JD