I know this is software and software is unlike other stuff, but if we were talking about a sport shoe, car design or other 'thing', where a lot of time, effort

I know this is software and software is unlike other stuff, but if we were talking about a sport shoe, car design or other 'thing', where a lot of time, effort and money had clearly been spent then I would consider it unethical to copy it too closely without being clearly innovative in some area. It's not just an 'idea' that you are copying, it's the actual design and UX implementation. Imagine you were talking about MS Word or Photoshop...