No it does need to be endless because it doesn't take 50 consecutive days, it takes 50 cumulative days
Oh. I thought it was consecutive, and if you were interrupted at any point at all, it had to start over.
That certainly changes things, yes. I can grant that if it is cumulative,
eventually the party will be able to make something, assuming they travel to places that have the tools they need to continue their work. (As an example, if you were making a sword, most villages would probably have a blacksmith, so if you have your own smithing tools, you could spend a day here, a day there, etc. But you couldn't necessarily count on every village having tools, supplies, etc. for making a silk robe or a glass staff, for example--and even bringing your own tools can only get you so far.
Still, even 50 cumulative days could take quite a long time IRL in many campaigns, especially if the game actually considers overland travel time. I know far too many groups that just handwave overland travel as though it were nothing. Can't really be forging and hammering and quenching your fancy magic sword if you're in a covered wagon for a week--and that's a pretty reasonable travel time between moderately-sized medieval towns/cities by horse-drawn wagon. Even purely on horseback, switching to fresh horses every chance you get, and moving at the maximum speed you can manage while still eating and sleeping, it would still take multiple days to travel even a few hundred miles. Modern "extreme" horse-riding competitions where you don't stop for 24 continuous hours (not going to happen with PCs) can barely make 100 miles, and leave both horse and rider utterly exhausted.
Point being: if you make it 50 continuous days of labor, it's a pretty difficult (not impossible, but difficult) challenge for most groups. If you make it 50 cumulative days, sure, it may eventually happen, but it could be months IRL, and by that point the character's needs or desires could have easily changed. This isn't some insane wish-granting engine where players just get whatever they want as soon as they want with no complications or consequences.
And folks wanting to curtail excessive usage simply need to add two relatively reasonable restrictions: one, as noted, make it require consecutive days rather than cumulative ones, and two, require that the PC(s) actually have the personal expertise to do what they want to do, or that they work with artisans who do have it. The former, as noted, makes it a dicey proposition that you can sit still long enough to get the task done. The latter makes it so you have to do the crafting in an actual workshop or the like, meaning podunk nowhere villages aren't gonna cut it, you need a relatively sizable town or even city.
@Hussar has the much more reasonable concern: "Oh great.
More spells. Just what I always wanted."