Well, I just wanted to show it is very much possible the protester you were referring to had a well thought-out position regarding taxation and government finances. All of these issues are complex enough that I feel I’d make a fool of myself trying to address each through a single sentence. I’m glad you personally have such a clear outlook on them though.

There are many talking points in France regarding this. Here are a few left wing ones one might hear at these protests:

• As in all Western countries, the tax system is effectively regressive, with the ultra wealthy (100M€+ in assets) paying only 10–15% taxes compared to 30–35% in the rest of society. The so-called “Zucman tax” sought to address this, but it failed in parliament in spite of very favorable polling across the citizenry
• President Macron has implemented a supply-side economic strategy with generous subsidies to companies. Left parties would use this money on the demand side instead (salary increases and such)
• Amidst cuts, the government has increased the army and police budgets, which the left sees as adventurism and repression.
• Pensions are very expensive and some want to reduce them (very explosive topic).
• This isn’t specifically left wing but the top levels of government are often seen as corrupt and money might be getting embezzled.