As you already know: All vent liners are not created equal. The current Lehigh (that we sell) is 17-4 stainless and heat treated/carbonited. Vastly superior to the old style Savages: Should give double or triple the life of the original black ones if not even more... Have plenty of folks (shooting blackhorn209) that report 300 plus shots and still can't get a .036 ZZ pin gage through our 17-4 Stainless Lehigh vent liners. At $5.25 each with as many as you want shipped for $7.00 against the cost of a new breech plug with bushing... Are you so sure its worth it for most folks to go with a new breech plug that has a bushing? I'm not convinced.
Yes loads, pressures, cleaning habits, primer choice (due to varying heat and amounts of ground glass in the priming compound) all affect vent/bushing/flash hole erosion/life. Whether its a vent liner or carbide bushing...
If I was shooting a rifle every weekend and/or shooting matches with it, then I see the advantage of a carbide bushing: long life and huge periods without measurement/maintenance would be very beneficial there. In that case one could even shoot enough to show that the bushing saved $... but I think the bushing is just a smart move for folks who shoot so much they could wear a vent out in a day or two of shooting. And that is a small portion of shooters.
But for most of us who are just hunting and playing with loads to get our gear ready to hunt, again, I'm not convinced a bushing is better than a vent liner. Seems like the simple effective solution is just getting good vent liners (like our Lehigh's) and a pin gage and checking the flash hole size every time one does maintenance. They'll probably last a few seasons before most folks get enough shots on them to replace them... and the replacement is cheap, especially if you buy in bulk.
My .02 USD... who am I as a guy who sells stuff to challenge a super user/mod/admin on the forum, with a different perspective?
Tom