Every couple of weeks somebody asks me whether it makes sense to move from an older Sun SPARC machine to a new x86 machine running either Linux or Solaris. My usual answer is that it depends: mainly on what the thing is supposed to do, how it fits with the other stuff you have, and who looks after it.
This week, however, I want to look at various aspects of that question and see if it's possible to offer a clearer and less general answer.
In most cases the driving force for this comes from the fact that support costs on older hardware move in the opposite direction from purchase costs for warrentied hardware of comparable or greater processing power - but the primary issue in making that change comes from the need for architectural change imposed because the current version of what you have in place has moved too far upscale relative to your requirements.
Basically, as your hardware gets older your monthly support costs become an ever increasing fraction of the cost of replacing it with comparably powerful new gear - new gear that typically comes with a year or more of warranty coverage as good or better than the coverage you're paying for on the old gear. At the same time, however, the fact that the applications still work on existing hardware means that the replacement hardware comes from much lower down the relative performance scale - and adopting it can often require an architecture change.
On net, therefore, the incentives sometimes work for change, and sometimes against - and sometimes they collide in unpleasant ways.
One former client has an application, for example, that's both mission critical to his business and essentially unchanged for over ten years. It currently runs on a pair of Sun 450s from the Solaris 2.7 days - and while he could get better performance at near zero incremental cost by swapping in a pair of otherwise retired Xeon servers running Linux, he has no incentive to take any risk because the thing's success makes it invisible to user management and he stopped paying Sun anything in about 2003 when he put a decommissioned, but fully functional, 450 away as a backup.
Another has huge incentives to change - but can't for internal political reasons. On the incentive side they're currently paying IBM more per quarter in support on a couple of P690s and a shared disk store than it would cost to replace them with brand new T5440s complete with three years of gold support - and on the negative side their original ERP decisions were so bad (customization and best of breed) that the company nearly went under getting anything working, and so user management will now reliably throw panic hissies if anybody in IT so much as thinks about making any changes.
And yet, the simple bottom line is that both are going to have to change - it's just a fact of life: costs change; gear becomes obsolete, old software becomes a drag on positive change, failure risks increase as equipment goes past its engineered end of life.