This is generally a bad idea in any field. You want your lower level gameplay to accurately represent the general form and concept of higher level gameplay. If you don't, you'll get all the "mass carnage" people joining, loving it, and quitting as they get to endgame, and you'll have all the "challenging single-target enemy" people quitting out at low levels due to the appearance that this isn't their type of game. Some players in both groups will investigate higher level gameplay, but that just means you lose a few of the first group faster, and force the well-informed members of the second group to slog through something they dislike. No-one wins. You have to make your game's intended audience clear and honest at all stages, or you'll end up with the wrong people, missed opportunities, and high attrition.
But that's why I said that you should still have a taste of the other side at all times. The Yin and Yang approach. And this is just in the context of solo pve. The game will be mainly designed for party play, so it's not like solos are even the target audience. I just suggested a way to hook them into the game more, because they'd still be able to farm easy group mobs at top lvls, but would still have to group up for more difficult content, same as they probably would have to do to kill hard singular mobs at low lvls.
I see it like this. Quite a lot of people will probably start the game alone and they might not like if the game is super hardcore right off the bat. But if they enjoy the game enough to get to higher lvls, to a point where you kinda need to find people to help you - there'd be more chances that by that time they would've met a guild they like or just a group of people to play with. And at that point that can try out the more difficult content, which would mainly be solo mobs (in my preference that is, I dunno how it'll be in Ashes).
If they don't like that gameplay, they can still go farm group mobs, but now there's fewer places where they can do that. So they might still have to group up at least with 1-2 people to defend themselves from other people farming those locations who might want to pvp them (that is, if the player in question doesn't want to pvp). So the difficulty of the overall gameplay still goes up. The risk vs reward equation stays the same. And this pushes people towards guilds/groups, which is the semi-final goal of the game because there's a tooon of group/guild gameplay, but you gotta be a part of those groups to experiences it and unless you push people into that situation - most people won't even try.
I might be wrong in my proposition of how things would go with my suggested mob balance, but I personally feel like it would be good, which is why I'd prefer it to be done this way.