I think those craving strategy were some of the earliest adopters of gaming, especially once those games became increasing popular. It's no surprise then that their numbers would be diluted over time, especially once you start including mobile gamers (who I think are different enough to not really warrant being compared to other gamers). As someone who played some strategy games in the 90s, it was a wild time:

• real-time games like Dune, Command & Conquer, Homeworld, Age of Empires, Myth
• turn-based stuff like Ogre Battle, Fire Emblem, X-Com, Jagged Alliance
• the ungodly amount of grid-based civil war and cold war games and the beginnings of what could be called grand strategy, such as Panzer General, Romance of the Three Kingdoms, Star Wars Rebellion, Europe Universalis (2000 but not really a stretch to include imo)
• 4X games like Civilization, Alpha Centauri, Master of Orion
• stuff that doesn't fit in anywhere else like The Guild, Majesty, Carrier Command, Battlezone... (might be misremembering release dates here)

We are still getting a lot of good strategy games even in recent years, like The Last Spell, They Are Billions, Beyond All Reason, half the stuff SplattercatGaming covers...

I imagine that there is a lot of cross-over between strategy, city-builder, logistics and sim players especially if you single out Germany lol. All those genres are "shrinking" if you are only looking at them as a percentage of total gamers, but actually they slowly grow all the time.