While I would prefer a soldered on base over a dovetailed receiver ring, it was common practice for European makers as well as Mauser themselves to do this.
In the case of the very early '98s with the original Mauser scope bases, Mauser dovetailed the front base into barrel right over the chamber! This is the same system that Rigby adopted as their method of scoping. Never heard of one of those blowing either.

The Winchester 54 receiver ring isn't as robust as a large ring Mauser '98 so that may be a consideration although the rifle has most certainly seen use as it is.
I have no idea why a gunsmith would patently dismiss such a rifle as unsafe. If he understood the design of the bolt and action and how they work together he wouldn't be so dismissive.

It sounds as though the dovetail is pretty small if it is between the screw holes on the action.
The new base will obscure most of it and it really is more of an aesthetic fix than a necessary one.