Here are a couple of thoughts on website lifespan;
What is the lifespan of a website? 2-3 years
Why? In the ever changing world of the internet, the technology changes quickly and without notice.
The website standards change and companion coding changes (java, jquery, php versions etc.) The browsers change how they interpret the website coding so it is imperative that your website code is kept up to date to minimize chances of hacking, to give the best user experience, cross browser/ device compatibility and so much more.
To keep the code current, a CMS system like WordPress can be updated regularly and your theme can be updated or changed on yearly basis. This in fact is a much more cost effective way to update the look and code of your website than to pay for a totally new website each time. Establish a relationship with a web designer and discuss these things ahead of time. It will save you many hassles and money in the future.