In fact, if you consider "native" to mean "using the standard platform frameworks", then the stated proposition "Are native apps dead? Not yet… but they’re going to be." will be realized by Flutter.
You could say the same when comparing native iOS apps vs web apps, and these days almost nobody cares. This jankiness will be solved --wether it is by optimizing flutter's core rendering engine or just because mobile devices will receive more powerful CPUs.
One way or another you will be able to build hugely successful multiplatform apps just using Flutter.