See I don't think about it like that. I think touching on the Bucky/Nat history in this movie, might actually be a good foundation and opening for a Black Widow Origins movie. And until proven otherwise I will hold on to that belief.

I assume you have better knowledge of comics than I do (honestly, it's superficial), so you probably see more possibilities than me. I'm definitely excited to see more of Scarlett as Natasha, but I can't shake the fear that they've teased us with Clint/Nat and then changed their minds... IDK. I wouldn't like that. A storyline that opens possibilities for her origin movie? Sure. I would like that as well. It's just - when it comes to shipping (and when I ship two people really hard) I get all these anxious feelings. I can live with Bucky/Nat as something that happened in the past, and it's part of who she is, I'd prefer to have Clint in her present.

My knowledge is quite superficial too, but it's not a matter of knowing the comicbook storyline. It's a matter of storytelling in general.

From what I know about the Comic!Verse and in combination with how they've handled the MCU narrative until now I can't say I'm too worried.

Could your fears come true? Sure! But it's not the only way to go. And it's because Marvel so far has been pretty good with how they've dealt with issues like these, I'm cautiously optimistic.

I've had really bad experiences when it comes to PTBs and shipping and how character storylines are handled, which is why I have this irrational voice in my head. I tend to be distrustful, but I see your point. I agree with things you've pointed out - I'm just more skeptical, IDK. Pessimistic at times? Probably.

I hope MCU continues to do a good work like they've been so far.

I understand where you're coming from, because, really, who hasn't had bad experiences? ;)
And, in all honesty, I generally am of the mindset "hope for the best but expect the worst". It's always on the back of my mind hovering. However I need a reason to be actively put into that line of thinking and get all worried before I have to.

In Phase 1, Marvel has established certain things and laid other foundations for relationships and dynamics. And so far, they have followed through with pretty much everything. So they haven't given me that reason to have that attitude. I don't live in a bubble! I know what people do for the sake of "DRAMA!!!" I'm aware of it and I won't be shocked if it happens.
But judging by the studio's history I feel I can approach this in good faith.