There may possibly have been some confounding factors during the twentieth century though, like the hugely destructive war, the shift from empires being a benefit to a cost and their consequent dismantling, and numerous mass casualty and economic disruption events. Russia lost something in the region of sixteen million people to the war.
Also I think you need to be clear on absolute vs relative decline - the population of the UK hasn't declined in absolute numbers, nor has its economy, and whether you mean "size of economy" or "GDP per person".
He's talking relative. And it matters. For example stuff stops being sold/is never sold in relatively small markets. We're starting to see it with China-only products. There are many disadvantages to being a smaller economy, even if in per capita terms things are ok.
And almost all European countries, taken individually, are predicted to fall outside the top 10 by 2050 (moderate chance for 2050 and huge chance for 2100).
No, but it will mean they will go older, with less dynamic and more geriatric society, worse economic momentum, and so on.
Historically momentum (not absolute position, which depends on prior trajecory and other accidental factors) is tied to population.
When France ruled the world, it was the most populous nation in Europe (1/3 Europeans was French). When Europe in aggregate was the center of power (through colonialism, science, industry, etc), European countries were in the 10 most populous globally (along with China and India). The decline in influence of Britain, France, etc, followed their population decline - in 1950 still 4 of the top populated countries in the world were European (including Russia) and 8 of the top 20. In 2015 only Russia remains in the top 10, and soon there will be no European country in the top 20.
Similarly, when the US emerged and established itself in the international scene as a strong player and then a dominant one, it also got in the top list of most populous countries.
(Historically, China, with its huge population, was the strongest economy on the planet, until the "historical accident" of the New World in the 16th century. Now that the European momentum has stalled, it's getting back up there again).