Internet Browsers are no longer a standard, stable reference platform – there is now a wide choice available, from multiple vendors, with frequent updates and ‘developments’ often due to discovered security risks.
Hardware platforms too are constantly changing – Desktops, Laptops, Tablets, Phablets, Mobile phone… what works well with your website today might not tomorrow …
Search Engines in the main rank your website not just against relevance but also against revenue and advertising potential – stale content gets pushed to the back of the queue or listings.
Retention – Users like to see changing content in order to maintain interest – but not be overwhelmed with essays.
In a similar way to any asset in life, if you look after it it will reciprocate with long and useful service. With websites there are competing industries vying for dominance and market share hence platforms enjoy very brief periods of stability. On the other hand there is a cottage industry continuously looking for ways to exploit the gaps that are left open.
Regular maintenance, even the most basic, is essential for your website and other software driven assets.