As much as I might not like this, I can admit it. Considering the hand we've all been dealt I doubt STP will ever truly have another singer that fills all voids. While I have defended what I believe to be their right to use the STP name sans Scott in the past, I also as a fan feel that it truly isn't STP without even one of the original four guys.
As far as musical relevance goes.... If they ever do find a guy to sing then I feel as though their first single might attract some attention for a second from whatever is currently called "alternative rock radio". It would be more out of curiosity than anything though, kind of like why people slow down on the highway to see a car wreck. Time has passed STP by, and by just merely completing a formula I feel they have zero chance at musical relevance and being cutting edge in 2016 or beyond. They'll just be a nostalgia act that just so happens to have a new record.
Sure they will continue to play shows; probably some fests here and there and some House Of Blues sized venues or perhaps a little smaller as time carries on[size=78%], but that's about it.[/size]
Now...... I feel this CAN all change somewhat IF the band changes course and picks a girl. As much as I don't necessarily like Lady Gaga's music, I will admit, she would be interesting enough in a live setting to where this band would get some buzz and interest, I could also see her being beneficial for getting new material in rotation and product placement.
Or they could pick an unknown girl. That could be just as interesting artistically, though perhaps not quite as successful commercially. Yet an unknown girl could give them a little street cred again.
That's my two cents on the matter.