If Janis makes the opening week roster there´s no reason to keep another fast receiver to spread out a secondary on the roster.
I would draw a parallel to the DB situation in thinking 6 WRs is a possibility.
In the bread and butter nickel defense, there are 6 experienced players, all decent or better in one or more roles, even if Richardson is limited. Two high picks are the injury insurance or better.
In the bread and butter 3-wide offense, there are 3 quality starters with the backups having little or no experience, with the #5 options each being a low pick or UDFA carrying some question marks as to how they'd perform in money games.
From the perspective of competition, it doesn't end in preseason, and "next man up" isn't simply a shifting up of the depth chart when an injury occurs. We've seen Jones benched multiple games in the past after running a bad route and dropping a couple of balls. We've seen Bush installed as the opening day cover corner and then bounced out in short order.
In an injury situation, the first "next man up" might not cut it when the bright lights are turned on and the "next next man up" gets his shot.
Given that a 5 man group provides such little backup experience and less than robust resumes in one case, having only 2 in the injury-replacement numbers game may not be sufficient. The issue is particularly acute at wideout if the Packers are not of mind to move Cobb out of the slot role if Nelson or Adams were to go down. Then it's #5 or broke if Montgomery is the slot receiver we think he is.
Further, if the 5 and 6 guys happen to be Janis and White (or White and Janis), for example, White has no PS eligibility as you pointed out earlier and Janis would be a risk of being signed away off PS which was the logic behind keeping him on the active roster last season. All it would take is some team losing it's KO returner to take a shot at Janis with the stretch-the-field option as a kicker. In other words, putting him on practice squad presents the same risk as Charles Johnson.
The best case for a 5 man group is the organization not having a favorite for #6...in other words a guy they've concluded is not regular-snap material, such releasing White, sending Janis to PS because they've gone cold on him, and/or Abbrederis is still working off his injury.