If the penalty was always -1, for example, then it would be more pleasing and +40 more reachable. But I think we agree that the % rate is not the main issue.
Also, at the same time, equaling everything to 51% chance rate does not make lower levels easier, but certanly make higher levels more affordable (as everything is flat when you reach this rate) if you gonna compare to other games’ way to handle enhance system, specially if you gonna ignore both games’ “level reduction when fail” aspect.
The % itself is not the main problem, but the dificulty aspect of ToS enhancement system. Following your example, every single attempt have the same chance to be true or false, but what makes it hard is, if false, you will loose 5 other lucky attempts you got. Fail another time in a row and then 10 lucky attempts are gone. It is even worse when you can only control the ammount of attempts (through different anvils), not the luck itself (like an anvil that could increase the success rate)
As % rate is flat, this level-reduction introduces the dificulty aspect of the system, that is not progressive. You can feel very frustrated at +21 for example, where you should not because you reached only half of the maximum level you can get.
This is not creating a progressive dificulty that could be rewarding at lower levels and risk-taken at higher ones, it is creating a burden. Today, a +40 equipment is considered cheating due to how frustrating a person would get to ever attempt reach this level