Well... (noting the date) not anymore at least.

aka, try stealing, and you get arrested.

Profits aren’t somehow defined as theft, despite all value being produced by the labour of workers, who are paid a fraction of the value they produce.

only if you believe that the capital being used to produce value - including the wages being paid upfront - are valueless.

Also, does a fixed initial investment entitle one to a fraction of all future (variable) value produced? Why?

The new capital is _produced_ by the workers, but it was done with initial capital that is _owned_ by the capitalist. So unless you agree that these initial capital is not worth anything (despite it being required), how can _all_ capital belong to the workers that made it? After all, the workers agreed to receive their wage in exchange for producing, using the seed capital supplied by the capitalist.

Yes, this appropriation is central to the capitalist idea of ownership. Another word for that is theft.