The rules have already changed, and 90 day life batteriers are compulsory from the end of the decade. Many companies have already incorporated these into their products as they come through for scheuduled servicing...
The drawback to a transponder is the added complexity of the recieve / process / transmit circuitry needed.
Many companies are already looking at ejected CVR / FDR, or even a simple floating transmitter with a GPS locator built in.
There is always a cost/benefit analysis, and ultimately the number of aircraft lost in this way is tiny in the grand scheme of things.