Google acquiring YouTube also had good synergy because it caused bandwidth costs to plummet, which YouTube was having a hard time managing as a startup. The company was bleeding money on bandwidth, because they were an end user who had to pay an upstream ISP for transport [1]. Once Google acquired them, suddenly they're on effectively a backbone network and have settlement-free peering with all kinds of other networks. That drove bandwidth costs down to near zero according to one analysis [2].
Also, "just give it as much resources as it needs, we're rich" was a game google had already proved willing to win with gmail.
It's easy to lay the tactic out in retrospect. Fund "resource hogs" that users don't pay for. Bet on long term bandwidth costs going down. Bet on major consumer monopolies being valuable, long term. Sounds great and it was great.
OTOH, lets pour $mns into a "business" that we bought for $bns, that has no revenue... because in 15 years we will be worth $trns and it will all sound like peanuts... this was once considered imprudent business planning. Google were willing to do it. Others weren't. Only a few even could.
[1] https://arstechnica.com/gadgets/2015/04/cheaper-bandwidth-or...
[2] https://www.wired.com/2009/10/youtube-bandwidth/