Ruby on rails is a large piece of software and while it makes it pretty easy to make good choices when writing your website, security problems do crop up. These problems need to be patched just like most other pieces of software.
If these security problems are not fixed then there is a chance that your website could be attacked, hacked, broken into. This could result in your website going offline or worse, customer information being stolen.
Web hosting technology evolves like other software.
The servers that your website is hosted on are upgraded over time. There will come a time where your website isn’t able to run anymore because it’s too out of date.
New features and technologies are brought into the framework all the time.
Rails 5 brought channels and web sockets which make asynchronous communication between client and server easier. Rails 5.2 brings Active Storage which makes working with cloud storage providers such as Amazon S3 a lot simpler.
If you allow your ruby on rails code to get too far behind the current stable version getting access to new features may become too expensive.
As a site ages it can become difficult to get the code running on a developer’s computer.
The harder that process is the more likely it is that developers will choose not to work on your code because it is too much effort to even get started. This could mean your site fails further behind or becomes vulnerable to some security issue. Ultimately the site might have to be written again from scratch wasting your previous investment.