Ask HN: Starting same business of the company I left?

How morally wrong is to leave a company and start the same business because you like the business, you think it can be way better and you know that that company will never improve it? What could be the problems? How could I get the customers?

The story behind the question, if you are interested:

I worked for 2 years for a small company (25 people), in a small team (4/5 people, all pretty young, 27/33). Basically fresh from college, I was hired as a backend engineer to re-do the backend services (DB, APIs). Lots of decisions were already taken when I joined.

These decisions were not great: a lot of architectural choices were done with no rational behind, just by feeling. DB was a mess, no triggers, inconsistent data, zero to null security, and so on. Application layer even worse.

What's worse is that the dev team was given ALL THE TIME AND SUPPORT from the management to redo everything from scratch, but still made a mess, eg, again, de-normalizing the DB without knowing if it would really be a performance plus, but just because "I feel it would be".

I don't claim to be the best, but I think that as engineers we should at least try to take decisions based on real things, and not "by feeling" (which I could accept from someone with a lot of experience and recognized in a field).

I learned a lot, but lots of things I wanted to do, implement and improve did not get approved. So, at some point my I had enough and left, because I saw that the only things we were doing was trying to fix the mess that was done, and I didn't want to go on with that when clearly the whole codebase was so messy that the only solution would be to start over with a real and well defined plan.

The service now works somehow (with "duct tape", as we say) -although there are lots of complaints from customers- so management now is kindof ok with that (because they spend money and time in the redo, so don't want to do that again), although they know its shitty (once again).