Li Shenron
Legend
On 'I shouldn't have to be a rogue to be an assassin', that's what multi-classing is for! Be a rogue (assassin) 3/fighter X, and be a fighter who is also an assassin.
More generally, a game with class-based characters has the problem of unrealistic inflexibility; a problem that multi-classing solves. It's not a bug, it's a feature.![]()
But you don't need to go as far as multiclassing!

There are feats which grant another class' features, such as Fighter's maneuvers or anyone's spells. They have limits, and maybe there is no feat granting Assassin's abilities yet. But this is only with PHB feats, more will be coming in the future and you can always design your own.
But what is most important, is that the mechanics support concepts but very often they are not strictly necessary. If you think that the only way to represent being an assassin is having the Rogue/Assassin subclass' specific features, then you are creating your own problem. And by saying that multiclassing is the solution, kind of implies you are still thinking about that Assassinate feature as mandatory. It is not.
An assassin is someone who assassinates people, i.e. killing them out of combat or just before combat starts. You may need some mechanic that makes you good (or better than others) at that, but you don't need the Assassinate mechanic exclusively.