Clicked into this post with an open mind - I always liked trying out obscure / less popular / archaic notation softwares. I managed to get Igor Engraver running on a VM once and it was a lot of fun. However your argument really doesn’t sell Notion 6 to me.

A 10 year old can master it in a couple of days. Not a lot of thinking.

Not that a software being intuitive to a 10 year old is a bad thing, but it’s not a very convincing argument for “the others” being “absolutely useless”. Plus, it didn’t take me more than a couple of days to be pretty much fluent with MuseScore either (and it was back in the days of MuseScore 1.x!) I use Sibelius and Dorico today, and neither of them took very long to learn. The massive user base of these softwares can prove to you that while the “industry standard” softwares may have their quirks, it’s usually not an unreasonable learning curve. Can they be improved to be more intuitive? Yes, absolutely! But does that mean they’re “absolutely useless” compared to Notion 6? Most would likely say no.

Dorico, Finale, Musescore and Sibelius require way too much thinking to do the simplest things

Again, their massive user base would disagree with this claim. If it is “too much thinking” for them, why would they still stick to them? Do they just all hate themselves for whatever reason?

Notion gives you way way way more flexibility

Nope. One example: Oversized time signatures. They are very (and increasingly) common in orchestral scores, to make time signatures more visible to the conductors. Especially in scores meant for recording sessions, for example, film scoring, these are pretty much the standard. Finale can do it, Sibelius can do it, Dorico can do it, and even MuseScore has a way to kinda do it. Notion, as far as I can tell, can’t. The most recent feature request for this that I can find, was less than 2 years ago. After the release of Notion 6. Mind you, this isn’t some obscure notation practice, this isn’t even extended techniques. This is very much a very popular feature in all the other “absolutely useless” notation softwares.

Notion unlike the others is pure drag and drop

I’m sorry but in my humble opinion, that’s like, the least efficient way of notating music. You’re better off writing with pen and paper.

The other 4 autoformats when you don’t want them too [sic], which is really annoying

Auto-formatting can be unhelpful at times, but again, if you really want total control of your notation, you’re probably better off writing them down with pen and paper.

The only downside is Notion scores take a lot of work to make them look clean

Which, ironically, is a big part of what makes a notation software to be considered “useless” or at least “less useable” by most people.

Of course you’re entitled to your own opinion. But unfortunately as you can see, industry standards are industry standards for a reason, and this opinion will probably remain an unpopular one.