I've had to live with several different software packages that have gone out-of-support. You can get away with it for ages. Emphasis on "getting away with it".
The problem comes is when something out of your control mandates an upgrade. In my experience incremental upgrades are relatively straight forward. The task gets exponentially more complex with the generations skipped. In some cases the effort to upgrade is so great it is worth asking what we actually get out of the software and whether we should migrate to something better. Apart from the Azure/on-premise dynamic this doesn't really apply to SQL Server but ETL tooling, BI platforms it really does.
When my current organisation moved to the cloud quite a bit of software that worked on-premise would not work in the cloud or there wasn't an appropriate licensing model for running in the cloud. Cue big migration effort.
Migrations are not for the faint hearted. They are major undertakings and probably projects in their own right rather than part of a project or story. They can put you in the situation of being one of Steve's "Expert Beginners", starting from scratch now all your hard won knowledge of a toolset is null and void.
Being out-of-support isn't the difficulty, its when the route to an upgrade is blocked