Is there something better than old-fashioned glazing putty to seal
glass in wood sashes? I was thinking that maybe RTV silicone would do
better and last longer.

Any thoughts?

If you're still using the old oil and whiting putty, then yes. I've used DAP for years. Usually lasts until the glass is replaced. Key thing is that the glass has to be CLEAN, no fingerprints, no oil, no paint, where the putty is supposed to stick to it. Same thing with the frame, old putty has to be scraped off down to bare wood. Then a coat of paint over the top keeps it good for 20 years or so. I did the 25 windows(plus storms) the folks had during a college break one year, lasted until they replaced them with vinyl 30 years later. A clean and polished putty knife helps, too. Flexible for application, sharp and rigid for chipping out the old stuff.

Stan