...we will never see a period again - as we have in the last 50 years, where lanterns and columns have had service lives of 40+ years.An interesting point being offered by SilverRay.
In some respects, the longevity of the UK streetlighting infrastructure, is identical to other areas of "public" assets, which were also introduced to great aplomb in the postwar years as part of a huge period of modernisation. Comparable examples would be British Railways, the GPO, National Grid etc. All the obsolete plant associated with these sectors would have been replaced long ago, but due mainly to the lack of political will and funding, such equipment was destined to last a good while longer.
Other points to note however are that public equipment at the time was sold on its merits of not just being modern, but being economical, long lasting and cheap to maintain. This is why concrete was seen as the obvious choice for public lighting columns and why they lasted for so long. The same argument is seemingly offered when looking to install LEDs, however as SilverRay points out such equipment installed today won't have anywhere near as long a service life.
Finally, it must be remembered that we live in a modern world, where long lasting appliances are bad for the economy. It makes no sense to install equipment that will last for 50 years without a hitch, because the economy and nation needs to be fed. This difficulty however has been traditionally alleviated by employing two fundamental laws - obsolescence and fashion.
As has been pointed out already, a mobile phone is soon declared obsolete technically due to the continual improvement in technology. However, it can become out-of-date even if still working, simply because it is old fashioned or ugly when compared to its successor. This is why the market aims new products at the young, because they follow fashion (or peer pressure) and generally have the disposable income to fund it.
In terms of streetlighting where fashion and obsolescence is already playing a part, the new LED lanterns now gracing our highways, will themselves be scrap in a short space of time indeed - well, for as long as the new money continues to roll in!