And a counter counter
1. Yes a lighter bullet must be driven faster to achieve the required energy and yes some bullets may not perform as desired at close or distant targets. This unfortunately is unavoidable. As I know you are aware, all expanding bullets have a window they perform best in regards expansion. I personally don't feel that velocity is the answer to "what makes a cartidge great?". Meat damage being my main objection. The difference in drop is negligible once you know your rifle some.
The beauty of a popular round such as the .243 Win is that pretty much every bullet has been tried in the UK, on our game and at most sensible ranges. This wealth of knowledge leads people to choose their bullet with the amassed experience of several.
The less said about 'sniping' the better

2. Can't believe I said calibre, I must be tired today. It's a pet peeve when people don't say cartridge when it's appropriate. I would certainly not want to be anywhere near a bison shot with a .243, not my idea of fun.
Ever tried finding a 45-70 in the UK let alone bigger? The choice ain't great I can tell you

3. I must be VERY tired, I thought I'd typed .275 not .270. He did indeed use a Jeffrey double some (if not most) of the time.
He wasn't relying on expansion that much is true. I believe the idea still stands though as he was using what he was the most accurate and nimble with for the very specialised and dangerous form of hunting her used it for.
My point being, there may be better cartridges out there but .243 Win isn't inadequate for the majority of our species (although not all), it is just at the bottom of the power pile.
It does the job, no doubt about it. Those who want bigger, go bigger. Those that don't, just pimp up their .243.
Personally, I'd go .358 Win. But then again I am compensating....I am in the Navy after all