You can't be the best runner if you aren't the best at any one event. There is always someone better at any event you run. All that you describe is a runner with the best range. But that counts for less than being the best at at least one championship distance.
Bolt did not need to run the 400m to be the best ever sprinter; Rudisha didn't need to run the 400m or the mile to be the best ever 800 runner; El G holds the wr's for the 1500/mile (and did hold 2000m) and was Olympic champion over the 1500 (and 5000) and WC over the 1500. To have been arguably one of the best ever at his main distance didn't - and doesn't - require he excel at any other. A runner who excels at a range of events but isn't the best at any is always behind those who do - either literally or as they are measured historically. They haven't achieved what the best specialist achieves. To be the best - at anything - requires being a specialist.
You're just repeating what I said. Bolt is the best at the distances he runs, so is El G, the actual act of running that includes putting one foot in front of the other is the same whether you run 100m or the marathon.
If you can only excel at one specific distance then you're not a better runner than someone who can excel at several, you're just good at that one distance.