What can we expect?
Drupal 10 is already in development and is currently in the alpha stage as I write this. Projects are being worked on in contributed modules that once ready will be integrated into Drupal 9 before Drupal 10 is released so that the upgrade is easy. As with the Drupal 8 to Drupal 9 upgrade, Drupal 8.9.x being the same as Drupal 9.0.0, the Drupal 9.x.x last version will be the same as Drupal 10.
Drupal always had a bad rap about the upgrade process or lack of it but this has been one of the major areas worked on in recent years and so far the Drupal 8 to 9 upgrades I have done have been straightforward. Keeping Drupal 9 up to date will mean the same for the Drupal 10 upgrade.
Moreover, a lot of contributed modules were ready for the upgrade on the release day of Drupal 9. With one of the ultimate goals of Drupal now to support hassle-free upgrades and to keep the upgrades in sync with other dependencies, Drupal will be secure long into the future.
If you modify Drupal using contributed or custom modules, as long as it is well-supported or in the case of custom modules it adheres to modern coding standards, then you shouldn't have an issue when upgrading. This should be the goal of any software, so if you decided on some other platform or service, this is something you should consider.
Currently, a lot of the Drupal contributed modules are Drupal 10 ready so things a looking promising for the release of Drupal 10 in December 2022. To see the progress of module readiness Aquia has a nice dashboard showing the current Drupal 10 readiness status of 8630 modules that are tracked by the Drupal Association.