No monopoly. Only for cable internet, which may be a possible argument. For landline internet (DSL), there's plenty of alternatives.

Unfortunately, having a landline capable of DSL is no longer the default in Germany.

Some apartment buildings exlusively offer DOCSIS via a single provider (as there's never been any unbundling of the DOCSIS "local loop"; presumably under the assumption that a landline will always be available anyway?).

If that one provider is oversubscribed, you're pretty much out of luck.

High speed internet is a market not just internet access. Email might not care that your on a DSL connection but a streamer can’t generally use DSL as a substitute.