That's fair enough. I thought it could be a way of getting around the whole 'They made a 10 Might weapon. Well, I'm gonna make a 20 Might weapon.' 'Oh, damn, 20 Might weapon! I'm gonna make a 30 Might weapon!' as it would make the system less straightforward than pure Might or pure levels. But I agree that it would overcomplicate things if we try to define that all in some way. Fuzzy and ill-defined is best in this case.
Weapon inflation is a self-solving problem. I kinda suffer a similar effect of it with the Divas, like for example, making Chronicle (8 might) instead of adding that might to the Holy Site I was making on that turn has a clear effect on my might expendability. Since I had to numbercrush a bit so I can guarantee that I can get the Mind domain ASAP, the effect of that 4 might per turn was clear to me when I made her, and it isn't nice number-wise, even if absolutely necessary for a better story.
With that in mind I can only imagine what a 10 to 20 might weapon could do in the long term to a player, especially since it provides less narrative possibilities than an avatar. Anything like a 15 or 20 might weapon not only it hinders the future might of the player, but it also isolates the player from the ongoing narrative.
And anyway, the people with the strongest weapons are also people whose characters are also in relative isolation (Logos and Niciel). I don't count Zephyrion's staff as the same as the other two because it was mostly a larger investment on Vetros, maybe Niciel's staff could have a similar effect.
Edit: The whole decision of making a 10 might weapon rests on the balance between the other options. For me for example, it would go like (Make 10 might weapon) vs (Invest in new lands, new species, new concepts) vs (Invest in might per turn generation). For the last turns I have mostly gone with the middle option, I will go a bit with the last one now because I need level 14 to have the Mind Domain, still, I numbercrushed hard to guarantee I can still keep adding a load of new things every turn instead of just banking on might.
Basically, do you think if Lugi was still around, Slough would have even spent 10 might on a weapon instead of bringing in new creatures/biomes? The way to stop a possible weapons race is by making that middle option more valuable in face of the others, and surely powering up might value per level isn't the way to do it, on the contrary, I few like it makes one less likely to spend on there, since in 4 levels or so one can make it all for free.