Being on an outdated version of Django can result in security issues. This could open up your website to attacks from hackers that might bring the site down completely, or cause your customers' information to be stolen. The ramifications of this happening can mean lost revenue, loss of trust from your customers, and even lawsuits.
Ease of Maintenance
Doing any new work becomes much harder. If you'd like to add future features, the time to do them on an old version of the code means they will take longer and be more expensive.
An outdated Django version can negatively impact the customer experience, resulting in user frustration, an increase in customer support, and lost business.
Skipping an version upgrade means that the next one will be exponentially more difficult and take much more time, costing you more. Some libraries that are integrated into your system may become incompatible with the old version, meaning you will have to update those libraries to new versions and that takes time and money.