Why not? Soloq is a unique and fun bracket which tests your ability to play in any comp against any comp. Your ability to adapt to situation and carry your teammates. Even though luck is involved in matchups, it's statistically impossible to have "bad luck" over hundreds of games in the same way it's impossible to get carried every time. The whole probability of getting a good/bad matchup gets averaged to roughly 50%.
In the long run, you are the deciding factor and your winrate is derived from whether you constantly bring your team down or help them win. As you move up the ladder, you are matched with better and better players and the same logic applies. Thus, soloq is competitive by definition.
Now, it might not be as competitive as 3s, but it certainly is infinitely more popular for many reasons - you can queue at any time, you don't need to synchronize your timetable with your partners, you don't get bored of it, because you are constantly presented with new situations and people.
In my opinion, soloq is the only thing that keeps people playing on Blackrock if I don't count the random battleground heroes. Which brings me to your second point that I strongly disagree with.
Removing the rewards is a great way to kill the last remaining active bracket on Blackrock and finish off the server for good. By removing the rewards, you will successfully take away the very thing that motivates the majority of the players to queue in the first place. You probably think they will be forced to move to 3s, but most of them won't. Instead, they will leave the server. It's never a good idea to force players into something and it will always generate a backslash.
To illustrate, out of the average of 50 players that are active in soloq at any given moment. 30 will leave, 10 will keep queuing until the bracket inevitably dies of the lack of popularity and the remaining 10 might try to do 3s. But they will need to find partners and synchronize with them to be able to queue. So in the end, you might have generated perhaps one or two active 3s teams at any given moment. And I wonder how long will it last before they find out the bracket isn't active at all and leave as well.
I guess it's alright to increase rewards for 3s such as more frequent flushes, less rating for gear and so on. But don't do it on the expense of soloq and don't make the rewards bracket-specific. We've seen it with wrathful shoulders, now we are seeing it with pve shoulders and heroic weapons. It just doesn't work.