If they're Afghan or Iranian - whose countries will not take them back - where do you deport them to? France? Where they entered the EU? I can't see the French, or other EU countries, going for that.
eg If I (British citizen) arrive at a French port having already used up my Schengen limits I'm forced to return to the UK.
If an Iranian citizen arrived ain UK (with no valid asylum claim) then as an Iranian citizen you put them of a flight to Iran.
Remember we are talking numbers here. Start timely processing of asylum claims from France and the majority will chose that path rather than risking their lives in a small boat spending a fortune they don't have.As number drop to low so the business model of the people smugglers is destroyed, no longer worth them getting boats there, etc. for so few people. So the route is stopped and maybe the few whose claims are rejected and can't be returned will have to try and find other means? But numbers will be dramatically reduced. Boats will not be sinking mid-Channel with the deaths, etc.
Also, the majority of people applying are having their asylum claims accepted. It's the minority whose claims are rejected, then from that minority many will likely be from countries they can be returned to, etc. So the number of failed asylum claims who can't be returned becomes very low.
I'm still undecided about the Government's suggestion it might stop all visas (or all types) for citizens from countries refusing to take back citizens whose asylym claims are rejected. I've not heard much discussion about practicality, negative aspects, etc. so I'm still open minded on that.