I get the whole "don't give monsters abilities that you don't want the players using" style, but having a dragon mouth deep throating an arm before you chomp it off seems like a fair limitation on the maneuver.
If the lich gets himself temporarily snuffed out and then reforms from his phylactery, he's got both his arms good as new, right? If so the whole dismemberment thing could become pretty common for the wimpy level 6 lich. Maybe some residue of whatever the dragon did (especially if the dragon gets away alive,) leaves his arm easy enough for less serious opponents to detach, though in those fights he could reasonably retrieve it and stick it back in place to resume his spell slinging- sort of an excuse to make him or other party members run around a little more, or run away and lick their wounds while they wait for him to reform.
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I think the reason that you don't see a lot of intelligent and long lived monster types being bajillionth level anything is the same reason that not all humanoids ever gain many levels. These adventuring parties generally skyrocket in power the way they do because these adventures range from remarkable to amazing. You don't level up just for existing and breaking the necks of some little rats that scurry around- you level up because you're putting your neck on the line and pushing the limits of what's possible.
Moreover there's some clause about not rewarding xp for fights that flat out didn't represent any challenge, so especially with intelligent evil NPCs the whole keeping themselves in positions of relative safety is going to keep them roughly in line with how much power is typical for the type of monster that they are. With a number of beasties we've got several entries that kind of track your typical X as they grow older- so that pretty much gives you the baseline for power gain without taking big risks and generally biding your (unlimited) time.
For the most part monsters have stats xyz just like common humans in town have stats abc. As wild and evil as the land may be it generally supports a much lower population of monsters than the bustling population of humanoids in their cities, or probably still lower than the population of some quiet rural farmland. The monsters tend to be bigger or at least meaner, but based on the total number you should have even fewer that serve as the equivalent to adventurers.