Unfortunately, the calculation of 8 years needed for civ7 is quite sensible, as much as I hate to type that
Civ5 needed 5 years of development, while Civ6 needed 6 years. However it was before the pandemic, which disrupted the industry a lot, and could reasonably cause many months of delay in productivity.
And there is one more thing: civ6 was quite conservative iteration - it was much more conservative transition than massive jumps between previous civ games. Put final versions of civ3, civ4, civ5 and civ6 next to each other, look at the screenshots, and see how much civ6 looks like "civ5,5" both at the first look and in terms of the fundamental mechanics. And we all know many of those mechanics are really old and need more depth - religion, trade routes, the way production and gold and tiles and pops work, 1upt combat, the way map works, and so on.
If we assume civ7 is going to shake up the fundamentals much more than civ6, more like civ5 did with civ4, to solve some fundamental problems of the game and introduce a lot of fresh air and brave experimental solutions, it needs more time in the oven.
To sum up, 6 years of civ6 development + disruptions caused by the pandemic + newer games generally needing more time to develop + civ7 needing to be more 'experimental' and risky than civ7 while delivering quality = 8 years of dev time don't sound that crazy.