This can very easily be solved by making it inflation adjusted (i.e. the value increases with inflation every year). This happens for instance with many social contributions in Switzerland and is IMHO the only reasonably way to set such value.
If you set UBI as inflation-adjusted, then there is no viable political mechanism to reduce it. "Cut everyone's income!" is a losing political message, forever ever.
Inflation is the pressure release valve that allows quietly cutting real costs. Every employer, business, government relies on this. It's why the Fed targets low but positive inflation.
I don't know. In Europe almost all of these things (welfare, social security, minimum wage) have been tied to inflation since forever. Sometimes they get cut, sometimes they get raised.
In Belgium a lot of stuff (salaries, rents, social welfare) is tied to inflation, and sometimes we do 'Index Jumps', where we simply don't index for a year.
Belgium is fairly unique in that respect. If I remember correctly, it also takes into account the evolution of salaries in neighboring countries (France, Germany) when deciding collective salary increases.
