Fine. I'm dumb and ignore everything because no other information than kegs are always the one true solution is impossible to compute. Ignore everything else. Always build kegs. Nothing else. Ever.
No, you're not dumb, and nobody said you were. But in cases where total profit is our main driver, which is surely the purpose of your post, your implicit assumption that there is some sort of capacity limit on processing simply isn't valid in
most cases, and I feel it has lead you to a false general conclusion. If there is any capacity limit at all, it's not time for the machines to process and it's certainly not space to put machines; as already pointed out, for the former, any such time limit is arbitrary and self-imposed, and for the latter, with the enormous amount of total space available across the SDV world, you will run out of growing space long before you run out of space to put machines.
The ultimate limit, if it existed, would in fact be this: whether you put a crop in a keg or in a jar, it's one interaction to load it in the machine, and another interaction to collect it when finished, and after accounting for the time needed to plant and harvest, your limit is the number of interactions that can be performed in the time available. And given that it's also impossible to run out of time to load and unload machines before you run out of crops to process, (because even with 7 days in a keg, the growth time of all the most valuable crops is still longer than the processing time), so you're financially better off getting as much money as possible per interaction. And the fact is, for the more expensive crops, you simply get more money per interaction when you perform those interactions with a keg than with a jar. The amount of time between those two interactions is irrelevant unless you have some sort of niche requirement for getting the cash exactly four days faster, but in all honesty, once you reach a certain point, you'll always have all the ready cash you need, negating the benefit of getting your artisan goods four days earlier.
For the cheaper crops, if you really want to process them, then sure, stick some of them in jars instead of kegs, because it's better money per interaction; personally I can't be bothered to process the likes of parsnips and blueberries at all, because of the lousy profit per interaction compared to the boredom factor of maintaining machines, and I'd rather chase excitement in Skull Cavern, but that's just a personal play-style preference.
TBH, the top performing veggies, like pumpkins, cauliflower and red cabbage, are so close in price from kegs and jars that it's barely worth worrying about either way as long as they get processed. But given how expensive jars are to make in comparison, and given that the real big money items, fruits like starfruit, ancient fruit, pineapples and melons, are simply so much better in kegs, I mean literally hundreds of g better, jarring them is basically burning money.