If you want to be precise, the imperial system looses that first advantage.

Well, this is clearly not true... you can measure just as accurately in either system... the fact you can convert between the two systems shows you can represent all the same data with both systems. The only thing I can possibly imagine you mean is if you using fractions only with imperial, but, you don't have to do that, the fraction side of it is useful for sure, but you aren't limited by it.

For the second advantage, try dividing by 3 two times in a row.

Sadly children these days, who never experienced the imperial system, probably consider this complicated, and can't fathom how imperial would make this easy. But 3/2 can be expressed simply as 3/2 if you like, so literally no work to do, and if you wish to divide by two again it just becomes 3/4 (double the bottom part of the fraction). You see, that was incredibly easy. Do the same in decial and you have 3/2 = 1.5 and 1.5/2 = 0.75 - which is, quite obviously, harder/more complicated. Just for a bit of fun, lets divide by two a couple more times: 3/2 -> 3/4 -> 3/8 -> 3/16 -> 3/32, let's do the same with decimal, 1.5 -> 0.75 -> 0.375 -> 0.1875 -> 0.09375. Hopefully that helps you.

Or if you want to divide by 3, so say we have 2 divided by 3 = 2/3, divided by 3 again = 2/9, divided by 3 again = 2/27. Let's do that with decimal, 2/3 = 0.6667, divided by 3 again = 0.222, divided by 3 again 0.07407. We are already at the point where I am guessing most people wouldn't be able to do this in their head... those who can then not particularly quickly... or those who can do it quickly, still, less quickly then just multiply a whole number by 3, which is what you do with fractions.

And, if a country goes full metric system, carpenters could still know inches.

Well, as the rest of your post hasn't really proved anything, this final remark doesn't really mean anything either. Of course the country could change, of course life would go on. But you say carpenters coudl still know inches, the problem is, the carpenter goes to the store to buy a door, and it would be in metric instead (this literally happened when vegetables switched to being measured in metric, and some people, again mostly older, went to the grocers and struggled to work out how much to order). Does it mean people can no longer buy vegetables, no, they can adapt, it isn't the end of the world. But my post was simply highlighting some of the reasons why imerpial isn't all bad - and the lack of understanding (shown in your example of not seeing how easy it is to divide by two twice) just leads to people having a big opinion about something they don't really understand, which isn't ideal.