The 100-400 has been around for, well forever it seems. It actually launched back in 1998, long before the dominance of digital. It’s also unique in the Canon lens range in that it is the only lens to use a push/pull zoom mechanism – the new one apparently won’t use this system.
Canon Rumours say the replacement will be “quite a bit more expensive than the current and ageing version”. So, better start saving, it’ll be on my shopping list.