You're not in IT are you...
It has little or nothing to do with the OS being bloated... This is an issue effecting ANY OS.
You replace old kit because:
- Application support - staying current in the business world with apps often means upgrades to hardware as well, and drops in productivity due to slower machines and less productive employees, or worse, and app upgrade requires an OS upgrade to be installed. Upgrading in place costs time, not only for the IT person, but the employee who's machine it is. In business, a typical PC based employee costs $50/hr, and the IT guy servicing it costs $100, (total business cost, which is not just labor and benefits, but equipment, space, heat, electricity, etc) then there's the load on ancillary systems and other support personnel. Also, cross supporting a current app one more than one OS increases testing cycles and complexity of the environment, as does having to maintain multiple different hardware images and drives bases of the same OS.
- Hardware maintenance - old parts mean that MTBF grows narrower. Every hour of downtime for an end user is lot time for the company in terms not only of that user, but someone on a help desk line, and someone else who facilitates the repair. Machines out of warranty not only cost time, but parts too. The frequency of failure only increases with age. 1 or 2 repairs in a year cost more than en entire new deployment.
- Employee perception. Yes, investment in hardware has a direct effect on employee morale. Using kit at work thats older than your $400 BestBuy special at home just feels wrong. Virtually ANY time, even a few seconds, spent waiting at work for an app, especially dozens if not a hundred micro-increments of time, simply drains on stamina, and it not only less productive itself, but frustrates employees and makes them actually do work slower as well.
Throwing a few hundred dollars into the budget to replace computers on a 3-4 year cycle keeps employees happier, lowers application testing costs, lowers support headaches, eliminates replacement part and upgrade costs, and eliminates LOTS and LOTS of help desk labor time (and staff). If you can eliminate just 1 help desk staffer, you could offset between 750 and 1,000 desktops with the money saved in just one year. More modern OS on more modern hardware also has backend benefits too.