I prefer "becomes untethered."
Reasonable speculation is closely based on the evidence (is closely tied to the evidence).
Bold speculation moves farther from the evidence (but is still tied/tethered to the evidence).
Pure speculation disregards the evidence (moves completely away from the evidence, so untied/untethered).
"Becomes substituted" is understandable, but more natural is "substitutes."
"speculation too easily substitutes for evidence."