Earlier quoted context omitted.
A monopoly is not defined by the possiblity that there are other products available for purchase on the market, but rather how many products have been purchased and are in use. Using your reasoning, Microsoft could never have had a monopoly on Windows because you always could have bought a Mac.
That latter definition is in fact a reasonable counter-argument to Windows being a monopoly (people did buy Macs), and that's why Microsoft got in trouble for the deals they cut to try and crush Netscape, not for making a more popular OS than Apple did. You can't define a monopoly as "your competitors aren't popular" because otherwise it'd be illegal to invent new product categories, as at the start you'd be the only…
As the sibling comment rightly says, monopolies are acceptable. But, as the EU clearly point out, a greater onus is put on monopolies to avoid abusing their monopoly power.