im one of the under 40s but ive seen black sabbath a few times, motorhead, judas priest, deep purple, ted nugent, etc etc as well as the majority of big name 80s thrash bands, punk then lots of death metal and some black metal you likely havent heard of.
it is really interesting to see bands that blow away your expectations as well as bands that i loved in recorded format but were awful or annoying in concert. i saw judas priest when they were in their mid 60s around 2009. obviously expectations were high but they still blew them the hell away, tons of encore songs and banter. it was amazing and unforgettable. megadeth is disappointing in concert lol i saw that so many times as a co-headliner i stopped caring at all. gwar was one of the top 3 concerts. i remember children of bodom exceeding my moderate expectations a lot, alexi laiho was a true musician. lol probably a lot more but those come to mind.
I keep saying I need to see Priest one more time before they call it quits. You just convinced me to check them out the next time they're in my area. In a similar vein, Iron Maiden has been consistently good live even as they've aged.
The one time I remember seeing Megadeth they headlined a 4-band lineup, and absolutely got blown out of the water by all 3 opening bands (Fear Factory, Flotsam & Jetsam, and Korn). They just kind of mailed it in, and played the exact set list you'd expect with zero deep cuts or surprises, and a rather vanilla stage presence.