I'm not making predictions on constants that we don't know; but when speaking
about exponential growth it is not even necessary. Want to know how fast the exponent
growth? Take your 50% growth, and just out of curiosity see for which n your (1.5)^n exceeds
the number of atoms in the universe. Gives some idea.
But the proposal isn't to exceed the number of atoms in the universe. It's to increase block size for 20 years then stop. If we do that starting with a 20MB block at 50% per year we arrive at 44,337 after 20 years. That's substantially under the number of atoms in the universe.
The point being, with an exponent it's too easy to overshoot.
How so? You can know exactly what value each year yields. It sounds like you're faulting exponents for exponents sake. Instead, give the reason you feel the resulting values are inappropriate. Here they are:
1: 20
2: 30
3: 45
4: 68
5: 101
6: 152
7: 228
8: 342
9: 513
10: 769
11: 1153
12: 1730
13: 2595
14: 3892
15: 5839
16: 8758
17: 13137
18: 19705
19: 29558
20: 44337