Matthew 21:44 is almost identical to this, so in effect συνθλάω is word with a New Testament frequency of two, but a usage of one.Luke 20:18 wrote:Πᾶς ὁ πεσὼν ἐπ’ ἐκεῖνον τὸν λίθον, συνθλασθήσεται· ἐφ’ ὃν δ’ ἂν πέσῃ, λικμήσει αὐτόν.
This is not really a question, but let me throw up my thinking on the board for comment.
If we accept the very strong gloss of συνθλάω, the I find that this imagery quite counterintuitive. Falling onto a stone is not such a serious thing, but συνθλάω seems to be glossed as quite a strong word. "Crush" or "dash to pieces" is extreme imagery. With just one's own body weight, somebody would be roughed up and bruised (perhaps mildly, and perhaps severely), but would be unlikely to be "crushed into pieces". In other circumstances, it might be that the specific gravity (density) of an obect or how brittle it is may cause a more devastating result from falling onto it, but people's bodies generally quite well-padded and fairly resilient or hardy. In their entry for θλαστός the idea of being "able to be crushed or bruised" is juxtaposed with θραυστός "frangible", "brittle".
If θλάω (take a look at the LSJ entry) were taken in the sense of "bruise" rather than "crush", it would make more sense. The συν- could mean "in or from contact with".
The change in tense from the future, to the less likely subjunctive suggests that there may have been a "wow!"-contrast intended in this saying. Something like, "Run (jog) into a truck and you'll hurt your pride, but if a truck ever ran into somebody, then they'd be GAME OVER." If that's the case then a milder word in the first phrase might suit it better.