Leather shoes on Tisha B’Av

Hello Rabbi,
Let me start with a short and funny story – during my military service I was an armored fighter in a Beinish company. We were in a training exercise in the Valley in the sweltering heat of August. Tisha B’Av arrived and we, the Beinish, were given permission not to wear military boots but sandals. And so it turned out that throughout the fast we walked around happily in sandals, smiling everywhere. And then, a few minutes later, when a secular soldier came across us and asked how we were walking around like this without military boots, we would put on a serious face and answer that it was Tisha B’Av today and we had to torture ourselves.
My question is – I heard the Rabbi in his lessons bring up the “dogma of the swimsuit” – that the Sages did not say to wear a swimsuit, but to adapt the swimsuit to the weather, and therefore a change is considered to be wearing a swimsuit in the cold. Why can’t we say here that the Sages wanted us to torture ourselves, and for them it was torture to walk without leather shoes, and if so, today it would be torture to walk with army boots, and sandals, and Crocs (which are really ugly sandals whose only justification is their comfort!) would be forbidden on Tisha B’Av?
And similarly, the prohibition of eating meat during the nine days – if one eats “non-festive” meat (grilled, etc.) but simply a quarter of a chicken that is considered a healthy and nutritious dish and not necessarily joyful, why not allow that and prohibit ice cream and other desserts?