If it's welded correctly with a big enough patch panel, it can be stronger than it was new, so no the make it weaker argument doesn't hold water. If done poorly then sure, it could be a problem again later. If the other side is the same design (with that stud near the bottom which seems to be the fracture origin) I would go ahead and weld a panel on there too.
However, examining the whole vehicle to assess the rest of it does have merit. I would still repair it because the cost of the repair is likely to be less than the depreciation of it becoming scrap metal value when the crack completely fails, but the point is that at this point I would consider whether to hold onto it or repair and sell it soon if there are many other rust areas that are going to be a problem soon.
For example if you're still on the original brake lines, replace them before you drive it much longer. DIY you can do that for $100 (closer to $150 including soft lines) or less. Having a shop do it, you're looking at $1000+ considering you'll probably want them to replace the soft lines (at each wheel) while they're at it. This is in addition to all the other misc. things you may encounter with a 22 year old vehicle, things that you can repair yourself inexpensively but if you have to have a shop do them, can easily exceed the value of the vehicle soon.
I am in favor of keeping an old Ranger on the road if the engine is in good shape, and tranny too if not a manual. It's a nice size for occasional hauling, before truck bloat and extreme cost became a thing, and parts are easy to find.