Again, not really, in the DMG itself it notes in the brief little blurb in the book they threw in the rules for such things in order to help with everything from running games set in the Renaissance (as far as early firearms, though I sigh that they think the Three Musketeers counts as "set in the Renaissance", but that's a sigh for another conversation), to indeed as you note the "crashed ship has modern technology", to, as they say, campaigns that contain modern day elements. Proficiency in firearms is noted as something the DM can decide if it fits their setting or not, with the note that with most settings, it would not. In a modern setting, or one with modern elements, it's not hard to note that firearms, if common, fall under martial weapons proficiency like anything else that does.
In that sense, while there are problems, the proficiency problem isn't really at all one of them.
Now, they don't give you much beyond firearms and explosives. Vehicles certainly would be nice to have stats for, but 5e is, for better or worse the "handwave" edition, giving you the tools for boom boom sticks and kersploding things is pretty much a big part of the fundamentals.
Also, while, yes, like when in Pathfinder for instance, you make guns commonplace and a martial weapon it fundamentally shifts the nature of combat as far as things like how much damage anyone with martial weapons proficiency can do, for some reason guns don't ignore armor in the DMG outside of burst fire, so it's not as bad as it could be.
Beyond that, I mean, guns would certainly fundamentally shift the nature of combat, allowing the equivalent of mooks to do a lot more damage and make the whole thing rougher. It's a hilarious word to use in context of D&D, but that's just the realistic consequence of guns. Characters that are standing out and dealing with that would indeed be higher level, and the various martial classes should all get the ranged fighting style, at minimum, and while the monk's arrow deflection works just fine against guns under the rules, the damage it deals with should likely get an increase. Other changes would be needed besides, but it's not impossible to make them. Like I said up above, the nuts and bolts are there to work with.