Modern Content Management Systems are brilliant allowing you to redesign, rebuild your website without having to redo the content, unless...
If you use Joomla, Wordpress or another CMS you will be able to add content and blogs to your heart's content. By now you may have built up hundreds of pages of fascinating content, brilliant.
The way these systems are built allows you to create new designs, new navigation, you can complately change the way your website looks and works. The big bonus, you can keep all your content and use it in the new look website. This is one of the BIG advantages of using a well established content management system such as Joomla or Wordpress.
Now let me explain extensions, or widgets as they are called in Wordpress. These are little bits of software that add extra functionality to a website. It might be a clever animated menu, something that displays your twitter feed or hundreds of other functions. There are literally thousands of these available to make your website work that much better.
For some another advantage is ready made templates. I'm not a fan of these myself but I can see they have a place. Increasingly they come as a package with a bunch of extensions or widgets that work with the template. Seems to make sense and works well mostly and helps one produce a quick working website.
Now, all software needs to be reguarly updated, whether it's on your desktop computer, your smart phone or your website. There's lots of reasons why, it is just a fact of life.
Your Joomla/Wordpress installation must be updated whenever an update becames available. Primarily for security reasons. There's always a bunch of lovely people out there trying to hack into your site generally just for fun, seriously. The developers endeavor to stay ahead of them with new fixes and such. These updates should leave the way your website looks and works completely unaffected. But there can be a problem...
I have recently inherited a website, developed by another company that had made extensive use of off an shelf template package with extensions. It turns out that nearly all the extensive content is managed via these extensions rather than the core Joomla system. To put it bluntly, the company 'Yootheme' are now in a position to hold my client to ransom. As I previously mentioned keeping the software up to date is essential. Updating the core Joomla software is free and always will be. Updating an extension does sometimes need paying for. Normally one has the option to look at an alternative widget or extension. But if all your content is managed by one company's widget or extension you can, in effect, be held to ransom. You can't look to alternative solution, at least not easily. If that company decide to discontinue a particular extension that your content relied on then you really are stuck unless you can find someone to divise a way to extract your data in a useable form. Generally possible but can be expensive.
The answer, ensure that your website's prime asset, the content, is held as part of the core CMS system. Joomla and Wordpress are part of the Open Source movement so won't be subject to the vagueries of the market and ensure a long healthy website future. Extensions are generally commercial additions that come and go. Many don't survive from one update to the next as they become incompatible and are not profitable to maintain.