How formal is "otherwise" in daily conversation?
For example: "Thank you for letting me know. Otherwise, I will probably still be blind." Does it sounds formal? I barely hear it in TV series. I'm not sure if they use "Or else" most of the time or not.
Are these the only two words that fit this kind of situation?

Thank you.

Question

How formal is "otherwise" in daily conversation?
For example: "Thank you for letting me know. Otherwise, I will probably still be blind." Does it sounds formal? I barely hear it in TV series. I'm not sure if they use "Or else" most of the time or not. 
Are these the only two words that fit this kind of situation?

Thank you.

Answer

I don't see any difference between both. It simply depends on the context. None of them is more formal than the other. 
On a side note, with a native english speaker, you wouldn't wanna use such an expression as "I would be blind". It might sound a little unnatural. 
Also the tense you chose here "Future Simple". It would also sound more natural if you used a conditional instead. 
I would just go with : "thanks for letting me know, otherwise I would have done so and so" 
Check this page out, might answer your question! 
https://idioms.thefreedictionary.com/or+else
I hope that was clear. :)

Answer

Otherwise is not formal or informal as far as I am aware. I hear and use it often. I am a native English speaker from the USA.