I've always wondered, in such cases where you have an open core and plan to monetize on extra features and plugins; what do you do if the community just goes ahead and adds those extra features? Or someone forks the project and adds the features?

The extra features are practically irrelevant. When you're spending other people's money, $995 a year for an "enterprise license" is pretty much a no-brainer, regardless of the actual benefits. It'd cost more than that for legal to check that a FOSS license isn't going to present any issues down the road. $995 a year is chump change, even if all it gets you is explicit permission to use the code in a commercial product.

It's extremely difficult for most enterprise customers to donate to FOSS projects. Small donations just aren't part of the normal corporate decision-making process. Very few people within an organisation have the authority to just give away money. Spending relatively small amounts of money on products and services is an utterly mundane part of doing business. If you maintain a FOSS project and don't offer an "enterprise license" of some description, you're leaving money on the table.

There are several Sidekiq features others have added but I never trust them. If I am an enterprise I value reliability over price. I would pay a premium for something the Sidekiq owner worked on rather than a random contributor

That's called business competition. Stay in business by delivering greater value.

It's likely the maintainer is in the best position to start a company about the product to deliver more features faster and at higher quality along with more inherent trust.

As someone being in a similar situation, it's a really difficult business model. Unlike many other business models with a natural moat (i.e. by virtue of having all the data, being deeply integrated into organisational processes, etc), commercialising open source is a struggle because simply justifying making money already requires constant improvement.

Almost all businesses require constant improvement to be commercially viable. This is the common scenario.

Natural moats are rare and highly sought after and fought for, and is actually much harder to build than a typical business based on product value.

Are there more products with the same model as Sidekiq which have the same level of financial success and are one man show?