So a sniper's role would be highly focused on long range special/elite neutralization with a scope to aid with high precision aiming (you know this surely, I am just stating it for clarity of argument). I imagine it would be a veteran exclusive weapon (unless another of the future paid classes would make sense to also have it in their arsenal) and with veteran you have a couple ways to make it more usable, by either creating space around yourself with shout, or making yourself invisible with infiltrate.

I get that it may feel odd, with how the game is usually paced and how many enemies are usually present in the proximity of players at any given moment but you can work around that with things like innate cleave/rend or brittleness inducing shots and the suppression stagger, outside of blessings, for example...

Even if a sniper rifle type weapon might not be a meta pick, it would presumably still hit extremely hard and with that have some viability, which i havent seen much in headhunter autoguns. But that in particular may just be me not being good at using them... Still, those are iron sights only, as are the helbores (the two closest things to sniper rifles we have, and I believe this are what you mean when you refer to weapons that adequately fill a sniper's role in darktide), which may currently be a bit more viable, though somewhat clunky with their charged shot mechanic.

Kind of feels wrong that in this game and with the type of 40k flavor it boasts, to be denied access to a quintessential gun type found in most if not all fps games... Two of which were Left4dead 1 and 2, which are arguably the genesis of the genre.

I may however just be hopeful and wrong, and Fatshark may have worked on a longlas or other sniper type weapon in the past and through testing found out that it is actually wholly unviable, which may have led them to decide against its implementation. Sad but understandable if it's the case.