Small boat arrivals account for 0.42% - yes 0.42% - of total inward migration. The rest, arriving by normal means, is almost entirely ‘legal’.
Given that the obstacles - political, legal, practical, military, ethical..- to your preferred method outweigh the difficulties in hardening the criteria for ‘legal’ entry by at least as much as the inverse of that proportion, it’s not hard to see where change, if it happens at all, is more likely.
In any case nobody has ever pretended that the shoddy Rwanda deal will ever clear the backlog. Its only value, if any, is as a deterrent.
Except that it's not 0.42%. That should be 4.2%. Grovelling apologies for my geriatric mental arithmetic. My point stands, however.