Just how much better than a seat in Y do you want?

The reason I ask is because you seem to assume the next thing up is J/C (note: some airlines call Business C, but others including AA call it J and in the case of AA C is only mileage upgrade to business), but it's not on all airlines. Some airlines, such as BA, have Premium Economy, which is better than Y but not as good as J/C, but costs much closer to Y than to J/C. While it may not be J/C, if you can afford to buy it outright, it's much dependable than counting on upgrades being always available, unless all your trips are booked a long time in advance and with the flexibility of being able to select only flights on which upgrades are confirmable at time of booking. (Note: At airlines other than UA, this is a separate cabin and separate booking class, so it's not quite the same as UA's Economy Plus.)

The other thing is that for some of us an exit row seat in Y is already much better than average Y, and at many airlines the first level of elite allows you to prereserve those at booking. So while you may not be able to upgrade on every flight, you want to make sure you're elite as early as possible to be able to at least take an exit row seat as a backup. (And if you're booking flights after the initial upgrade inventory is gone and have to go on a waitlist, as a low-level elite count on at least some of your upgrades not going through.)