Completely disagree with the OP.

I have enough scar tissue to believe bad UI or even the one that's not pleasing or inspiring receives lot lower engagement from users than having a clean, snappy and beautiful UI.

The OP's claims can be easily disproved by contradiction. If UI improvements didn't make a difference, A/B testing on designs wouldn't have resulted in massive improvements.

Making design more accessible, and attractive often helps beat the asymptotes in user consumption of a lot of products.