It's maybe impossible right now but bairstow and root know one of them is going to be dropped at the end of this series, it's just too much pressure. They also no 2-3 bad tests after that will see either the other one get a shot or someone else entirely come in.
There are two spots, but neither Root could conceivably open I guess. I know where you're coming from though, and ding dong just seems to be a fixture when his form isn't all that and he does enough every now and again to cement his place for another 3-4 Ashes series
Two reasons this is bad, Broad was quite a poor bowler for a long time and I'm not sure playing him all this time was worth it. Anyway, he's not so good you'd ever be going "god, wish we had two broads.".
As I said earlier, Broad is another Giles, the selectors see him as giving balance by being able to bat and bowl. The problem is his continued selection is never going to give him a chance to find any form, and as you say he's not "so good......." anyway.
Broad (Tests)
Series 01-05 : 26 wkts @ 45.23 (SR 83.38, ER 3.25, BB 3/34)
Series 06-10 : 57 wkts @ 32.02 (SR 60.05, SR 3.20, BB 6/91)
Series 11-15 : 62 wkts @ 24.03 (SR 55.23, ER 2.61, BB 6/46)
Series 16-20 : 38 wkts @ 35.55 (SR 64.39, ER 3.31, BB 7/72)
Nothing special, one purple patch which included three series where he averaged 23.36 against Pakistan at home, 13.84 against India at home and 20.46 against Pakistan in UAE.
His figures overall are pretty ordinary, for part of a four man attack he doesn't fire often enough and his batting is down to a 24.63 average with only three fifties since his 169 against Pakistan 24 Tests ago. I'd send him back to his county for at least this series, if he hits top form then bring him back for (or during) the Ashes.
For a four man attack to work we can't be carrying someone who in three of the previous four series he took wickets, half of them came in one innings : 6/51 in 11 wickets against New Zealand, 5/69 in 11 wickets against South Africa and 7/72 in 14 wickets against West Indies - interestingly the only Test of the three we won was against West Indies.
The other reason to give Broad back to his county is so another bowler (or two) can be tried out, if Broad is truly our first choice and gets injured, who comes in? Bresnan probably, he himself I think is coming back from injury. And isn't Broad playing all formats for England? Surely another reason to give him breaks.