Where does the proposed bill state that only replicas of powder burners will be prohibited? Answer: it doesn't. Read the actual text, as quoted above. It clearly captures any sub-500 fps air gun that has a 500+ fps version.
The bill's purpose is to prohibit all replicas that look like real firearms. Replica firearms are not replicas of air rifles. Nevertheless, the current ambiguity of the language of the Bill ought to be made more clear in the months to come. The language of a Bill can be very different from the language of an Act, the final version that is passed into law. In any case, it's likely that this Bill will not be passed by the current Parliament given the likelyhood of Canadians going to the polls sooner rather than later.
On a more general note, the argument that the Daisy 953c looks like the Daisy 953 is a circular argument. If it's legal to own an over-500 fps Daisy 953 by virtue of having a PAL, would the same rifle in the Canadian "c" 953c version become a prohibited weapon, even though it only exactly mimics itself?
It's worth keeping in mind that it's primarily the "airsoft crowd" in Canada that seems more concerned with the related provisions in the Bill.
As I understand it, under current law air soft guns don't require the "orange tip" that's often required by law in other jurisdictions. This orange tip distinguishes an air soft replica from the "real McCoy," the genuine firearm it may replicate. The the UK, for example, air soft guns must have 60% of their visible surface in one of several specific colours -- orange, blue, neon green, for example.
For some details on pre-Bill C-21 air soft laws, see this 2017 video h ttps://www.youtube.com/watch?v=6p3JpvoWMpU
In Canada, until the changes proposed in the new Bill, the law has required air soft guns to be within a certain MV and energy range to be immune from being considered a prohibited replica. As a result air soft guns could look very "real" -- just like the powderburning firearms they exactly look like.
As easily imagined, the trouble with air soft guns that look very much like genuine firearms is that people in general and law enforcement officers in particular can't easily tell the difference, especially under quickly moving circumstances. The new bill proposes to eliminate air soft guns that mimic real firearms. Perhaps this will require future air soft guns to have "orange tips" or have an easily identifiable colour scheme similar to that used in the UK.