Why have 20 slices when 10 will do?
Majority of people online rarely play much past 10th level as far as I've read online at least. Which inevitably means a good 50% of their character's class features/potential go to waste. So why not make each level heavier and more noticeable.
All the while not increasing the HD of each level up, thereby escaping the HP stodge and making the game feel more meaningful. Characters will max out with 10HD, which both ensures epic fights and also a level of danger which feels exciting. That and they get to enjoy the entire complement of their class features in a much more manageable time frame.
Okay but...well, a few problems.
By removing HP scaling, you have now removed almost all effects of levelling, because 5e was explicitly built around making HP the main scaling factor of character and monster power growth. Either you keep 5e's current "level/4" scaling, which means you gain...not very much at all over the course of 10 levels. Or you double it, to compensate for the halving of the level space...which means bringing back the math of The Edition That Must Not Be Named. You also make characters incredibly, incredibly squishy. Having half as much HP, but not changing anything about monsters at all, would make anything high-level genuinely impossible to fight, with characters regularly dropping to 0 in one or two hits, and crits potentially causing
instant death, no saves.
You'll (almost certainly) start seeing issues of things capping out at level 5, because at least
some of the problem is not the number of levels to chew through, but a clash between what people think the game
should be paced like vs what is actually
effective pacing. Another facet is, some players (mostly a certain subset of DMs) don't really seem to want to ever get beyond even the very early levels at all. Squishing things down so level 3-4 is actually level 2? So that by the time you hit level 5 you have 5th level spells? A number of people will start avoiding levels beyond 4-5, because high level is a bridge too far, either because they want more "zero to zero-and-a-bit" play, or because they want something that actually has at least a
little bit of balance left in it and thus shun the highest levels of spells where the wheels really come off.
Plenty of people feel this sort of thing actually reduces the amount of fun gameplay experiences. 13th Age reduced things to only 10 levels, and...yeah, there's some weight to the criticism that 10 chunky levels
feels smaller than 20 thinner levels, even if they've been designed to be the same. Whether this feeling is accurate or not, it's something to consider when creating a design.
Exactly the same effect (apart from the lost HP) can be achieved without changing anything at all about 5e simply by doubling all XP rewards. Why go to all the work of rewriting the game and potentially making it like The Edition That Must Not Be Named when you could just...stick a little "2x" modifier on each character. Doubling XP gained should be trivial to implement. If you're really desperate for that HP thing, double monster damage as well (perhaps only for creatures above level 2?)
