I understand the sentiment, I really do, because I have also used this argument in defence of previous managers - but context is absolutely critical.
Under Jose, the press constantly hounded him for not being close enough to City. If that's the criteria for success, then I'd absolutely use the argument that our footballing structure does not allow for our manager to be successful - crucially if winning the title/Champions League is the metric for 'success'
Unfortunately for ETH, who I was in favour of appointing and do like as a character, we're not even discussing whether we can compete with City for major trophies. We're talking about finishing top four. Progressing to the knockout stages of the Champions Leaugue. Having the odd deep domestic cup run etc...
If you use those as your 'success criteria', then it's absolutely possible to be 'successful', even allowing for the less than ideal footballing structure we have in-place currently.
For me, I don't care who is sat in the boardroom, there's enough talent in the squad to be performing much, much better than we are - both in terms of results, which are not there, and also performances, which are not there either.
There's no shame in moving on. ETH had a decent CV, he showed some potential, as it turns out, he's fallen short of elite. So move on and try again.