Apple insist you take upgrades don't they?
No.
All upgrades are written for the newest shinyest machines, and bring old ones to grinding slowness.
I'm running El Capitan Beta 3 on a 2007 iMac and a 2010 Air and it's extremely good. Beta 3 appears to have a smaller memory footprint than Yosemite.
I don't understand why it doesn't have any tools like defrag to clean it up.
Because like most Unix systems, the filesystem is nothing like as prone to fragmentation. If it puts your mind to rest, unplug your Time Machine backup, perform a fresh OS install and then reload the Time Machine backup. Half an hour's work plus a few hours unattended while it reloads the data. However, if you're doing that, the solution for all older Macs to make them as fast as new machines is to spend 75 quid and shove an SSD in, assuming that it has the maximum amount of RAM in.
It is true to say that Macs are currently close to unusable with less than 4GB of RAM in them. But with the exception of a tiny handful of early Airs which we can probably discount, anything sold with less than 4GB of RAM which will run remotely modern versions of OSX can be cheaply and easily upgraded to 4GB with a screw driver (Macbook Pro, iMac) or a couple of putty knives (Mac Mini).