This is actually more substantive than for Smallpox because Smallpox is deliberately stored in a lab in case it is interesting (supposedly for defensive research, in practice for weaponisation) and so invariably every decade or four it'll leak. Guinea Worm isn't suitable for that, when they eradicate it that's the end of the parasite forever.
This is an infinite benefit. So to model that without getting nonsense out you have to use a discounting model where you arbitrarily consider infinite future value to not be worth more than, say, 10 billion dollars, and there's your answer. But that's a paper exercise, why bother? You could choose values that say a trillion dollars or $9.99.
It's really not that arbitrary. There are many ways of valuing this, sure. Here's a fairly straightforward one:
Say it costs you A per year to maintain current levels, and the yearly cost of capital to eradicate is X amount of capital * y% yearly cost per year (e.g. if you borrow X at y% interest, or if you have X but sacrifice y% returns by spending the money instead of investing it), and the lowest viable cost to save at least as many lives as the infection currently claims is B, then it's a question of whether A+B or X*y% is lower. Whichever is lower will allow you to divert more funds to other targets, and so save more lives (or do something else). Of course there are risks here, in that any of the variables can change, so if they're close it may be worth it to eradicate simply to eliminate the risks that the alternatives get more expensive going forwards.
But this is not a very hard thing to model. The reason we don't do that more often is that often there are many other factors involved, such as politics and emotions and who fights hardest to get funded.
Those models work very well on a short-term timescale. We have major upheavals every few hundred years. At that point, the fiat money in the bank account disappears. Once eradicated, the worm doesn't come back.
Instead consider if it would cost 1B$ to eradicate them. Still a good thing to do... but the interest from 1B$/year is about 50M$ (order of magnitude). You could keep guinea worms completely eradicated and give away 48M$ to other things indefinitely.
(This is effectively the same calculation and efficient business should make for most any decision. Is the internal rate of return on this decision good enough to earn the money to do it.)
Say you could have a slightly higher number of Guinea worms but stable for 1M a year. The 1M you save might save many more lives if used for something else with a higher impact.
At 28 infections per year it's almost guaranteed there's something else that's already much more efficient at preventing deaths.
The 28 cases doesn't indicate low importance, it indicates those are enough to re-seed the worm if huge areas if not vigorously acted on, every time they see a new case.
I'm surely getting some details wrong, but it sounds like zero for several years has to be the goal, or it'll rebound swiftly.
There's also an ongoing health cost - for example, if vaccines apply (not in this particular case but in others) then if it's controlled by herd immunity, then you have to keep vaccinating and suffer any risks and sideeffects while doing so - but if it's eradicated, then you can stop doing so and there's some health benefit in being able to skip unneeded vaccines.