Here's a 1940s planes panel, yeah you can upgrade most any of the avionics on most anything, and still be way under the price point of a new plane, and in many times have a better airframe.
As far as some of the other stuff, most older planes are built WAAAY more robust than the newer stuff, I've had a emergency landing in a 40s plane and due to the strength and backcountry friendly gear design it didn't even damage the plane or flip, despite being soft sand
As far as "envelope protection" and "hypoxia protection" all aircraft have airspeed indicators and altimeters, so yeah, but if you're someone who doesn't pay attention you can install idiot lights and beepers in most any aircraft, I'd wager that DHC-2 panel I posted will start flashing if he gets near VNE.
Lots of professional operators also choose older airframes because some have advantages over newer stuff, look at the payload/cost of a 550ed U206 vs a new one, or look at a A185F or a DHC2, there really is no direct new replacement, and their market values reflect this.
Lots of older airframes have few ADs, are overbuilt, and have better systems for a trained pilot, good example is electric flaps in newer stuff vs manual flaps in older planes, you'll have a hard time finding a experienced aviator who wouldn't want manual flaps, heck even the cowls, new planes just have a little door to check oil and add oil, many of the older planes it opens to display the entire engine.