He underestimates the collectors power
The collector would be truly unstoppable if he could use 100% of his brain. But without the indestructible armor, spells, relics, etc he is just a regular human and as a regular human he can only use 10% without modifications
Hypothetically if he did use 100% percent of his brain you could say he isn't human by that point
The mind would no longer need the body and you would be left with a floating orb of unbelievable power and no idea of right or wrong
But the collector prefers to stay as he is
To stay human
To be able to enjoy the multiverse
So anyway on to my actual point
If beldum needs a crystal of creation he can just get it from another multiverse
There's always a being of destruction
And a Crystal of creation
After that new universes are made