I've been dealing with Drupal sites for about a decade and my own lean, when not working with Drupal shops, has been with non-profits and small businesses. Back in the early days of content management systems, Drupal stood out to myself as a multitool in a world that was still learning about blogging tools. In dealing with the user demographic of small businesses and non-profits, there was always one thing in common: small budgets. We're not talking Whitehouse.gov, Energy.gov, TheEconomist.com or the NYSE. We're talking MomAndPop.com and SaveTheWorld.org (metaphorically; if the sites exist I'm not referring to them but the idea of them).
This runs counter to the technologist and, arguably, the majority of the Drupal community's bend: Upgrade, upgrade, upgrade. Generally speaking, upgrading sites keeps Drupal folk employed and pays the bills and, for companies with larger budgets, this is simply the cost of doing business. I've seen Drupal sites lag behind 2 versions for those with smaller budgets and that is perfectly fine, once you maintain the site and assure functionality. The question those with smaller budgets need to ask themselves is whether or not it's actually worth upgrading.
The key to that question is really how many third party modules a site uses and where they are in the newer version of Drupal.
Presently, I've been talking to a non-profit about their Drupal 6 site. A lot of the functionality that they want in their upgraded site revolves around enhancing social media ability as well as look and feel of the site - but these things are not necessarily requirements for an upgrade. Drupal 7 can and has always brought a lot with it and it's likely Drupal 8 will as well, but the general direction of Drupal has always involved the breaking of third party modules and major changes to the core of Drupal (thus, a major version release!). When Drupal 7 came out, the community tried to fix that with getting third party module developers to pledge that they would have a Drupal 7 version when Drupal 7 came out.
To this day, third party modules suffer alpha releases and in the odd case, complete abandonment. Upgrading to a new major release of Drupal when a module you need is still in alpha or beta is, at least on the surface, not a good idea. An alpha module that hasn't been updated since 2011, as an example, could mean... anything. It could be a module that just works and not enough people use. It could be abandoned. Or it could simply mean that someone didn't bother to make it a release version for one reason or another.
The more dependencies you have on your site that aren't in Drupal Core, the more you need to think about whether it's worth the upgrade.
Look And Feel?
How your site looks is not necessarily dependent on the major version of Drupal that you're using, though theming for mobile devices is easier with more modern versions of Drupal - and yes, more people are using mobile devices. That said, if you upgrade a site you almost always have to upgrade the themes - meaning it's going to cost you.
Abandonment By Drupal Core
Abandonment is a harsh word, but I mean to point out how serious it actually is. In January of 2011, Drupal 5 was no longer supported. It is a matter of time before any version of Drupal (or any other software, for that matter) is no longer supported. If it's not getting security updates or core fixes, it's not something you want to be using. Period. Abandonments are typically planned ahead of time to give people the chance to upgrade but some people don't know about it because they don't have dedicated staff that keep an eye out for that sort of thing.
With Drupal 8 coming out probably within the next year, there is a question as to whether there will be an announcement about abandoning Drupal 6.
Simply put, here's what you need to look at.
- Decide what you want of the site.
- Look at what third party modules are involved and how well they are supported. The 'alpha', 'beta' and 'dev' releases of modules are yellow flags and mean you have to assess whether the modules are actually usable - and supported.
- Look at when the newest version of Drupal is to be released; typically 2 major versions of Drupal are officially supported at any time. If Drupal 8 is coming out in a year, your Drupal 6 site will likely not be supported in a year (as an example).
At the time of this writing, it's a grey area: You may not need to upgrade from Drupal 6 and you'll likely have at least a year before Drupal 6 support is dropped. It might be worth patching things along and upgrading to Drupal 8 in a year or so. It might not.