How much speed metal can really be said to be transitional, though? It's often argued that death and black metal were logical progressions from speed metal, but that argument doesn't really hold up chronologically or aesthetically. A more factually correct reading would see black, death and speed metal emerging simultaneously from the same source material, with speed metal basically forming an evolutionary dead end. What you're praising are not so much transitional bands as they are hybrid bands that learned not only from their predecessors, but from the emerging extreme metal that developed at the same time.
Speed Metal hit its creative climax and had already began to erode by the time Black and Death Metal started to seriously emerge. This, combined with the fact that the first wave bands from these two genres (Bathory up to and including Blood Fire Death
, Seven Churches
, Morbid Visions
, early Sodom, etc) all had a strong Speed Metal aesthetic or are heavily influenced by it to me, made it clear that it was a transitional genre between the NWOBHM and the extreme. I could however also see how the examples above are a synthesis of multiple styles that had developed at around the same time period so perhaps "hybrid" is a more appropriate term in description of the sound. A bit semantical, but this I suppose could also apply to bands that tilted towards the Speed Metal end of the spectrum (Slayer, Destruction, Pestilence, Kreator, etc).
Like Hardcore that came before it, I think part of the reason why Speed Metal reached its "evolutionary dead end" quickly while Black and Death Metal thrived is its shallow or really lack there of an ideological/philosophical basis. The other two had a much more richer palate of possibilities to work with from the inside out, so to speak.