Ok so Kennedy won’t work, (unless Tom become the roving player, I’m not advocating that but it didn’t play out to badly in origin) anyway it wasn’t so much about Will as the combination of $ available by letting Suli and Walker go, then using those funds to purchase a marquee signing that could give us a suitable cover for Tom if necessary and the option of having that roving role for Tom to link up with similar to how he played linking in with Teddy and Mitchell during origin.
You've quite rightly identified Kennedy as a wonderful footballer but no salary cap exists where you buy someone else's FG fullback as your backup option. Backup options are quality wingers that are happy to slot in at 1 where necessary or a rookie 1 from U/20s. The exception being Hynes who was a late bloomer and a manufactured 1 by a super coach and who now is collecting a payday elsewhere because Melbourne's cap can't afford him.
Raiders - Rapana (winger)
Dragons - Jack Bird (terrible option) or Tyrell Sloane (debuted for Dragons)
Panthers - Crichton (debuted for Penrith)
Broncos - Niu and Isaako (both debuted for Broncs; one centre, one winger)
Roosters - Manu (centre and debuted for Roosters)
Rabbitohs - Johnson (winger and debuted for Souths)
Titans - Jayden Campbell (debuted for Titans)
Cowboys - Jake Granville (hooker)
Tigers - Mbye (utility)
Eels - Will Smith (reserve grader)
Warriors - Walsh (rookie with 9 games to his name)
Bulldogs - Meaney and Allan (alternating given neither have nailed down the spot)
Sharks - right now they have no one with Dugan and Moylan injured - Mulitalo?
Knights - Tex Hoy (reserve grader that debuted for Newcastle)
It's clear that not one club has purchased a serious fullback from another club to be a backup fullback. You could argue Dugan, but he actually went backwards and has played mostly centre at the Sharks after his days as a fullback perished.
So, the idea of spending money to buy someone that can cover Tom is not a luxury any club can afford. The reality is, you have your best 1 and you do whatever you can to keep them on the field. When they are injured, you have to hope for a decent winger or up and coming rookie to fill the void.