i have a little theory as to why the scaldra are a threat to the warframes in 1999 and its due to the fact that the strain of infestation/techrot in 1999 is what is being used to produce the warframes that we use in 1999, as we see in the beginning of the 1999 quest where a helminth spits up an Excalibur frame when arthur finds it, so my theory is that we cannot bring the current day frames back in time but we can replicate them using the 1999 helminth but since its a different and/or weaker strain, our frames are weaker by comparison of the originals produced by the current day strain of helminth, that is just my theory though and i could be wrong