Speculation is part of the scientific process, everybody would agree. Fewer admit that philosophy and religion always play a part too, even if only because humans are not very objective critters.
It's reasonable to distinguish theories than have not been physically proven from ones that have. It's even more reasonable to separate theories that cannot be physically proven from those that can.
There is absolutely a difference between a current working theory that fits observable facts "well enough" for some, versus a theory that can and has been physically proven by experimentation and observation.
Proving a small case that requires a large amount of extrapolation on any dimension won't do. Ask a cosmologist what dimension or quality he is sure is linear at extreme values. Extrapolations that require millions or more years are a joke. Is "time" itself even "linear"?
There's no such thing as a physical theory that can be proven. Evolution is supported my more accumulated evidence than any other historical physical theory. It is impossible to accept geology without accepting evolution. It is impossible to deny geology without denying physics. All the physical sciences are co-joined.