I dove with J-Valves also....but once SPG's became a viable option it was a no brainer to move forward with the tech.I dove the rule of 120 for over 2 decades, and we guessed at our depth. Red ribbons never fail!
As for SPGs, I dove with J-valves instead. No bladder, either.
I also grew up as kid using dead reckoning and chart plotting to navigate the ocean outside the Golden Gate... Then they came out with Loran C and I thought that was the cats pajamas. Then they came out with GPS and that was the cats tuxedo. Point being that if the electronics ever failed, I still had some basic manual nav skills that might pay off.
As far as the topic of relying on a DM goes, what if the DM has a heart attack and dies? What does this group of totally reliant snowflakes do then?
I get following a DM. It can be totally relaxing sometimes to just kick back and generally follow the group. But I still monitor my computer and my compass to confirm that the DM is not putting me somewhere I don't want to be.