Sure, look, it's no problem if you disagree with the premise of the thread. But let me ask you this: If your opinion is simply "it's not broken", why bother coming on at all just to say that?
The converse could be asked of you. Why would you start a thread on a public forum about proposed house rules and reject outright any criticism of your premise (that the class is "broken" to begin with)? You are rejecting ways that the champion is equal or better than BM as if they are things that never happen. If they don't happen at your table, then awesome...ignore them.
If you're goal is to rewrite the class with like minded folks, recruit a handful, start a private thread then publish the results to the community (or not and just use them in your own game).
You're expansion of crit range idea won't be earth shattering. Take a monster with 100 hp that a fighter hits half the time....
Assume 1d8+5 damage per hit, with a crit on a 20 it will take about 20 attack rolls to take it out (9.5 average hit, hit half the time, +4.5 average per crit, crit 5% so 9.5*.5+4.5*.05 = 4.975 . So with a little luck it will take 20 attack rolls, probably 21.
crit on a 19: 5.2 per attack roll so about 19 attack rolls, maybe 20
crit on an 18: 5.425 per attack roll, ~18 or 19 attacks
crit on a 16: 5.875 per attack, ~17 attacks
Edit: The swinginess of the dice will probably hide or overwhelm these effects most of the time, same as with the use of SD against monsters with lots of hp. /Edit.
So basically with each you are making it more and more likely of taking out the 100 hp foe with one, possibly 2 fewer attach rolls. The use of each SD probably gives you about a 50/50 chance of using 1 less attack (this is a guess...someone better at statistics can do the math). So a crit range of 16+ is the same as using about 4 SD. This assumes using them as straight up damage or giving you one extra attack. Probably the best use of SD is to give the rogue an extra attack to get SA damage. At 17th level...that could be 10d6+5 (or so) extra damage...or it could be none if he misses or had some other way to use his reaction to attack or wants to use his reaction for something else.
For monsters with fewer HP/easier to hit the effects will be less pronounced. For monsters with more hp/harder to hit, it will be more pronounced.
For each +1 damage you give, that's about the same as 2 fewer attack rolls in the scenario above. For lower hp monsters it's less pronounced, for higher hp, more. (10.5*.5+4.5*.05=5.475 or ~18 maybe 19 attack rolls, the same as a crit range of 18+...if you want the champion's extra damage to be less swingy, +1/+2 seems about right to me, anything more and it is too much, I think)