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.
The worst thing is that you gave them the permission to send emails to you, but you did not even notice it.
If you give some information to any for profit company you can be sure they will use that information and if you decline the permits they are going to keep asking until you eventually miss click.
For example if you fill shopping basked, but abandon it after filling your information they can contact you once as "a friendly reminder" about the cart and they can keep that information legally for few weeks until they must anomize the data. And if you at some point clicked something where you accept the marketing permits they can keep that information "as long as the company thinks it is reasonable to keep and/or revelant information for their operation."
Source: Im part of the problem. Atleast for now.