One point I neglected about Tiktaalik is that its ability to walk on land was limited. Its limbs wouldn’t have been able to support it terribly well to do terribly much. Its life was likely spent more in the water than on land.
Coupled with the recent discovery of tetrapod footprints in a marine environment, the way to think of all this is that tetrapods did evolve at least 400 million years ago, but there were clearly still viable alternative lifestyles to go alongside fully terrestrial life (and still are). Nothing demands evolution be perfectly linear. (Neanderthals lived at the same time as our direct ancestors as recently as 30,000 years ago.) A further important fact is that while probably 90% or so of all fossils come from the ocean, they tend to be from the more settled sediments, i.e., not the shoreline, the evident habitat of these newly discovered tetrapods. That indicates a possible sampling bias. Just looking at Tiktaalik, it’s clear that its freshwater habitat lent itself to preserving fossils – aside from the area being targeted for its fossilizing properties, there were several examples extracted from the site.