There's definitely a danger if attestation becomes widespread enough that they can require it.
Not a danger of being unable to mod the apps, but they will be able to restrict access to their servers to the official unmodified app, when it's running on specific trusted operating systems.
This is already quite easy to do technologically, it's mostly a question of at what point Google feels it's worth doing, since once they start they have to commit to closing whatever exploits people find. And deal with the fallout of blocking a bunch of people on random old devices that weren't blocking ads anyway.
Of course people can still work around by running modified apps on rooted devices but it'll be enough to defeat a probably fairly large slice of users too lazy to jump through hoops - and as a bonus it won't just block Revanced (which is a fair bit of work to get running already) but also the other apps for media players like Smarttube, which were easier for people to set up.
And finally when all else fails they will spend the compute to embed the ads in the video stream, once they work out how to minimize the distribution costs for that.
Yup. I remeber the girls at school listening to bootlegged YouTube videos from shitty rip off apps from the appstore lol. Before revanced, there was vanced, after revanced, a new Phoenix will emerge. The people will it.