If this were the system you may as well display the number of matches a player has played rather than a level. Without the current achievement system I think we will see a decrease in the number of mobulas, squids, spires; and as the xp system is purely derived from wins, more frequent lobby stacking.
Honestly I don't think achievements deserve much credit for ship variety - at least in its positive form. Pretty much every enjoyable mobula, squid or spire I see is flown by someone who likes that ship and is good at it. The ships where it's flown because the pilot is doing it for an achievement and/or doesn't know what they're getting into
detract from my game experience, I don't know about yours.
GOIO has a joy in its variety but that is something in itself. It doesn't need people to be incentivised to discover it, it IS the incentive. It's pointless, it even deprecates from that variety, to push players towards specific loadouts. It's awesome that there's a huge choice, you can fly one of many different ships and then from there discover an endless depth of potential strategy and hilarity through guns, tactics, crew loadouts, your ally's ship and composition, playing on different maps... pushing players to sample the variety in little pieces, to give up their freedom to choose in order to maximise their progression is, in my opinion, not a positive effect.
I did actually address your concern about lobby stacking. Matchmaking is coming. Lobby stacking won't be a thing - or if it is, players will have no choice or ability to control it so it's irrelevant whether or not the XP encourages stacked lobbies. Because it's impossible to intentionally create a stacked lobby. Even if we ignore matchmaking, I'm sure it's not beyond Muse to build a system to gauge the "difficulty" of a lobby/team to decide XP rewards. simply averaging the levels of players on each team and multiplying winner's XP by the ratio of Loser:Winner, while rough, would be effective enough to make stacking generally unrewarding.
As for Dementio's argument... achievements don't
enable you to do anything. They
encourage you to do things. Don't suggest that players wouldn't try unusual builds or tactics without achievements because that's simply not true. I think imagination and seeing other peoples' strategies are far more important in creating new and interesting situations.
However the unfortunate case with achievements is that the rigid, linear structure of the achievements paths means that players are
encouraged to play in specific ways, before they are ready to try such a strategy, or when such a strategy is plain BS (eg. gunner with a Mallet or gunner light flak achieves).
You don't NEED to level up, no. However you don't NEED to play the game, or NEED to do any of the many things that you choose to do in your life. It's all a matter of encouragement. Hunger
encourages rather than forces eating, however it's widely acknowledged that people aren't just going to ignore hunger. The desire to level up, to find some way of progression within the game, to acquire a nice number by your name that stands as an instant and clear testament to your experience... yes, it's a much lesser force than hunger. However it's evidently pretty important to a great number of people so I think it stands that it shouldn't be just dismissed. It's not at all a case of "Yes the system has some weird consequences but you don't NEED to level up so it's fine". We can't ignore the significance of the levelling system, its importance and power has to be acknowledged and I think it would be a great benefit to the game and its community if that force was harnessed for good rather than left in its current sorry state.
I think the core, greatest problem of the achievement based levelling is its inherent selfishness. In an XP based system, a victory would result in a benefit for the whole team - your teamwork has achieved mutual gain. I've already explained how I feel this would also encourage players to discover the importance of teamwork sooner as it is by far the strongest way of playing to win. In the current achievement based system, at best, a player quietly and privately finds a ship that suits the requirements of their achievement and gets to work. At worst, the player will demand changes to the ship or make choices for their own loadout that reduce their team's efficiency, refusing to co-operate with other players because they won't progress if they do. This is a really bad thing in such a teamwork based game. I feel everything should be done to encourage teamwork and intuitively get across to new players the most fun and effective way to play the game whereas right now the level system, such a key motivator for some people, is optimised to be entirely divisive in nature.
The fact of most achievements being no fun, while in my opinion entirely true and yet another reason to scrap the whole system, is nothing like as important as the role of the levelling system in influencing how people play the game.