Richarlison's played on both flanks for us this season and if Son were to get injured or dropped, Richarlison would be next in line to start there. If we had Trossard, he'd represent the best alternative to Son in that position and on current form would probably start ahead of Son if we're being honest.
As for RW, there is in fairness little evidence he'd be great there but if we're in a situation where Kulusevski and Gil (and Richarlison?) are injured and you have to select Lucas or Trossard, then it's Trossard all day long.
He upgrades on Sessegnon blindfolded, I assume it was Sessegnon you meant instead of Emerson anyway, easy mistake though, both shocking players.
Richy has primarily been used as a RW, which is easily remembered as that's when our creativity started to disappear which may have had something to do with Deki being dropped and Richy taking his place, which once again has us lurching back to Conte's self-sabotage
As I honestly can't recall Trossard ever playing as a RW, it's really not a good bet to presume he might be decent cover for Deki, especially with our track record of having various tactical brainfarts along those same lines (i.e. because Aaron Lennon can play as a LW, move him over there to justify paying £15m on David Bentley)
The thing with Emerson and Sessgnon is our fanbase has made them Sacrificial Trash this season, which is why so many comments about them are interchangeable, hence Backup Emerson
...before you ask, Sacrificial Trash is a term to describe media where the following three steps occur:
i.) It prominently features female/minority/LGBT+ involvement in the cast or creative department that the angry CHUDs of the internet rant and rave about for months before it comes out (i.e. Ghostbusters reboot, The Force Awakens, Spider Man: Far From Home, High Guardian Spice)
ii.) It is not of brilliant quality when it does come out - which is why Far From Home and, to a lesser degree, The Force Awakens slipped out of those conversations PDQ
iii.) As a result of the first two points, discussion of its badness gets grossly overinflated - hence anyone has heard of High Guardian Spice in The Year of Our Great Old One of 2023
And that's what discussions about Emerson and Sessegnon are at the moment: people acting like they're Velma, to the point I've genuinely seen people call Emerson our worst-ever signing, when the reality is that they're 6/10 players in the current system yet people are acting like they're the antichrist