I don’t know how France classifies “income”, so it could be good at capturing income. It’s our own fault when something that is obviously “income” doesn’t technically count, in principle a tax system can capture everything that makes sense to count.

In the US, along with wages, interest income and dividend income also count as “regular” income and are taxed appropriately. Capital gains is… weirder and this is the first area ripe for opportunity to reform to capture “rich guy income is different than normal guy income”, as long as it is intelligent about it (e.g. if you said, without qualification, all capital gains are taxed like crazy, then suddenly selling your house as part of moving becomes an unreasonable burden, which is why it already has an exemption, but just an example to say vaguely why we have to be careful about capital gains).

Then you get to the borrowing you mention, and I’ve seen a pretty reasonable approach to capture that as “income”, in theory: https://equitablegrowth.org/closing-the-billionaire-borrowing-loophole-would-strengthen-the-progressivity-of-the-u-s-tax-code/

TL;DR: Currently borrowing doesn’t count as realizing gains, change it so that borrowing counts as “selling” the stock, further mandating that the cost basis of all identical stock is a specific way rather than letting the shareholder pick and choose their most favored cost basis.

I’d be willing to concede some tax break on repayment of such a loan to reconcile “real income” being exchanged for it down the road, but at that point I think it would largely be academic because suddenly there’s no point in borrowing against the stock rather than just selling it outright.