Buy European

Monopoly literally means that single entity has exclusive right to the market. Like for example in Finland VR (valtion rautatie, goverments railways) or Alko (only shop that can legally sell spirits for consumers).

There is also term called natural monopoly where getting in to the marked is impossible. Like railways in USA.

Steam is neither.

There is GoG, Epic, itch.io, Microsoft store and like ten smaller stores that are straight competitors for Steam.

Being market leader does not mean they have monopoly and if Steam somehow blunders the goodwill they have with gamers and devs there will be alternatives.