We aren't looking at those old galaxies today. Due to the speed of light and the vast distance involved we are looking at the light from those galaxies when they were very young and early in the life of the universe (that's why we are looking there in the first place). That light is just now reaching us. We have theorized about white holes but never seen one. By looking at closer objects we already know what an old galaxy looks like. And nothing can by definition exist before the big bang. Because before the big bang there wasn't any space for things to exist in. Nothing precedes it.

The big bang wasn't an explosion in space, but OF spacetime. Anything that might have existed before would have been fundamentally different, and completely destroyed in the event.

From the formation of the CMB we know that the whole universe was a hot and dense plasma that cooled and became transparent.

Are you proposing that these galaxies existed before and external to the CMB plasma ball "big bang" that we came from?

As in, a bunch of matter appears inside of a pre-existing universe as a local big bang, whose galaxies spread out amongst preexisting galaxies from older big bang events?

Then you propose that matter which has been hoarded by black holes may be the source of the matter in subsequent big bangs, to achieve a steady state.

I like the idea you are proposing.

My biggest question is: why didn't our "bang" blow all of the older generation of galaxies away from it such that we would never see them? My understanding is that spacetime itself is what expands / inflates in λcdm. It does so faster than the speed of light such that there is material in our universe from which light will never reach us. It's very hard to see things outside of a universe that expands faster than the light we use to observe it. It's spacetime itself that's expanding, not just the objects moving apart.

But, MOND is MOdified Newtonian Dynamics, and currently doesn't work with Einstein/GR... at all! So, if MOND is right maybe we should expect a different mechanism than Einstein expansion.

Most likely we just don't understand what stars and nebula looked like or how they formed back before metals existed and so we don't know how bright these galaxies should be to begin with because we don't know how stars work without metal. The assumption in the paper is irresponsibly invalid, we can't just assume that stars back then followed the same patterns as stars do now. Stars form from nebulae because metals condense out and coalesem creating nucleation sites for mass accretion. Earlier generations of stars would need to rely on different formation mechanisms, and likely had a different size and brightness distribution. We won't understand these early stars and galaxies until we've been looking at them for at least a decade.