I'd rather replace the keeper and take more control of games, play a higher line and create more chances by doing that, than have a guy up top having the ball lumped at him from deep and trying to feed off long through balls and second balls 90% of the time.
The games we have dropped points in, mostly away games against teams that can effectively play out from the back and high press effectively is because we lose all control in these games because we can't play out under pressure well and struggle to effectively high press because we can't confidently implement a high enough line to compress the space enough.
The reason why we struggle with both those factors is mostly because of the GK. And an upgrade to a ball playing sweeper keeper will make a huge difference to our entire set up.
I'm as critical as anyone of how much De Gea limits us, but I think you guys are underestimating how much the lack of a striker also hurts us. It's not just about scoring goals (if that's all it was then Ronaldo would have been fine last season and Rashford could do the job), a proper striker will also play a huge part in helping us control games and link our entire attack together. Martial can do it sometimes (when he's both fit and in form), but otherwise we have to go back to Zlatan to find the last time we had a striker who actually had the all-round game to play upfront for us.
A striker who actually has decent movement, hold-up play, ability to pass and link with others, and pressing ability would be huge for us. Much like even the average goalkeeper in the league has much better all-round game than De Gea, even the average striker has a much better all-round game than our options (other than the rarely-seen good Martial). Both would make a significant impact on our general control, ability to maintain possession and build attacks, but the striker would have a bigger impact on our goals scored tally than a new keeper would have on the goals conceded (at least you certainly hope so).
Really we need both, as one without the other will still see us having significant issues.