You don't have to upgrade to the new .uk platform, the way the changes have been designed mean that domain registrants can stay on the .co.uk for as long as they want - there is zero chance of it being discontinued. Some users will of course opt out and not upgrade.
What customers can not opt out of though is the .uk taking over as the new primary business platform. Countries all over the globe are dropping their .co equivalents and soon we expect that all domains will be on the second level and not the third. By opting out of the upgrade in the UK there are likely to be some negative effects.
In this section we go through the possible negative scenarios of a site not upgrading to the main .uk platform.