What am I missing about bonds?

Hey there. So I invest regularly into index funds following the bogleheads strategy (not exactly, but more or less). I use the formula "120 - your age" to allocate some money into bonds, currently using Vanguard Euro Government Bond Index (IE0007472115).

My country offers transfers between funds without paying taxes, so they have this advantage over ETFs.

What I don't really get is that this fund has had a -2.33% annualized return in the last 5 years, so I'm losing money here and there. I understand that it could have a positive return as well (YTD is in fact +0.3%), but seeing all this, wouldn't it be better to just have that money into a money market fund and have a guaranteed +1.9% return to at least somewhat keep up with inflation, or just put it into Trade Republic or something like that and earn the 3% (minus taxes)? When could these bonds surpass the money market fund? When the stock market goes down? It's the only thing I'm a bit lost about...

Thank you!