But theses are emotional
appeals rather than having any logical - or business - basis. I don't doubt that emotion plays a big part in people's decisions about suppliers and purchases, but that doesn't necessarily make those decisions better.
Would you bring your vehicle to the repair shop, knowing that the shop's owner is driving his vehicle with broken lamps, no brakes and without the exhaust system ?
Strangely enough, the mechanic I use is quite similar to your description. He buys dying cars, drives them for a few months, scraps them and then repeats the process. His explanation is that the last thing he wants to do is spend his time maintaining his own car, as he does that all day. Seems reasonable enough to me, and I don't really care - he fixes my own car well and charges a price I think is reasonable.
In fact, what's important is that the mechanic has worked on similar cars, knows what he's doing, and charges acceptably - not that he owns the same or equivalent car as me or you.
is owning a website in the top 5000 in Alexa a way to judge a good SEO?
You should care about whether your supplier has experience with such sites or sites similar to your own, not that they own the equivalent website themselves.
And besides, by definition there are only 5000 sites in Alexa's top list, and millions of sites who want SEO. Alexa is a measure of popularity which has very little to do with SEO in the first place. SEO is usually about profit and ROI, which is often in conflict with pursuing popularity.
And if everybody, who knows what does meta tag mean consider him/herself as a SEO specialist and furthermore, is trying to sell advises, then the industry in general become a "murky industry", as you agreed above.
I would encourage anyone looking to buy professional SEO to look carefully at the credentials and experience of any potential supplier. While their Alexa rankings may not be the right question, you should certainly be able to see evidence that they are able to do what they are offering to do. And if there is insufficient evidence for your own tastes, then you can look for an alternative supplier.
As for me individually, I'm not pitching to supply SEO, and so I don't think my own credentials are relevant. In the context of this thread, this is an online forum, and like any site posters should be judged on their contribution. With all the contributions being free, it's down to individuals to judge for themselves whether to listen to any advice they find here!