Queue times are an issue for sure, but by making Platinum actually queue with Bronze, Silver and Gold, it will just remove the point of actually having those skill tiers in the first place + you will have Platinum players that may hover between Platinum and Diamond, they will absolutely CRUSH whatever players you have in Bronze and Silver. This is a terrible user experience to go into and defeats the point of having low skill tiers.

The problem with ranked is that there is simply no reason to keep playing if you can't reach Platinum, obviously. But the way to address this is by making players organically interact with Ranked by giving them a reason to keep playing and rewarding them with specified Ranked Currency for winning at all ranks. Metal tiers also have terrible end-of-season rewards in general and even in gem tiers, Augmented Hellhound weapons are really not something greatly sought after, most of them look unfitting/ugly for the average player.

Your suggestion will allow for more queues, but it is just a band-aid on a gaping wound, not a cure - if you don't make time investment into Ranked appealing, you will always have low participation AND poor player retention (i.e. stops playing after 1-3 weeks), which is what the ACTUAL issue is.