5e is very stat dependent, every check is an ability check and bounded accuracy means your ability modifier is a significant part of that. When proficiency tops out for most things at +6, the possible +5 from ability mod is huge.
It is also more important in a single player game because, the star of the story needs to be doing almost everything. The star needs to be diplomatic, stealthy, athletic, knowledgeable, perceptive, you know just about all of it. That is another reason I recommend the bard class, the jack of all trades ability helps cover all the bases and the expertise ability lets them focus.
You kind of don't have to do everything. That is to say you don't have to do everything in the most direct and simple way. There will almost never be a problem that requires you do it in one and only one way, so the player should still have the ability to try and play to their strengths. It's just they will be doing some actions different then normal. Bard is a good class though.