1. "Brokered convention." If none of the current GOP candidates reaches the threshold level of 1237 delegates to be the nominee, the GOP will have an OPEN convention, not a "brokered" one. "Brokering" implies party elites override the actual delegates and choose the candidate. That can happen in the Democratic party, because the Democratic party has "Superdelegates," who make up some 15% of the delegates. The superdelegates in the Democratic party include party chairmen, all sitting governors and congressman, as well as former presidents and ex-presidents. All of these can vote freely, unbound by what their state delegations do -- so they really could, working together, broker a deal about who their nominee is.
It doesn't work that way in the GOP. First of all, there are far fewer super delegates -- only 3 per state delegation, about 7% of the delegates-- and they are bound to vote as their state did, so they're not really very "super." The result being that if the GOP gets to the convention without a nominee, there will be an exciting and fun open convention with lots of wheeling and dealing before a nominee emerges -- but undertaken by the delegates themselves, as representatives of the people. The process might be ugly, but it will be "honest," not controlled by party elites.
2. The process is rigged because of rule 40. Nonsense. There are as yet no rules for convention 2016, because the rules committee hasn't met yet. Follow the link to see how how the rules committee members are chosen to see why when there is an obvious party nominee, his people have virtual control over the rules (which is how Mitt Romney's people made rule #40). With no obvious nominee, the rules will be more of a scramble, and that's why the candidates are lobbying for rules in advance.
3. It would be an unjust establishment trick for rule 40 to be abandoned. Again, there is no rule 40. The rules committee will have to decide which rules carry forward to today. Ted Cruz desperately wants rule 40 (which says you have to have won 8 states to be eligible for nomination) to stand because he believes he'll get the necessary eight, believes he can take Donald Trump in a one-on-one fight, and he knows he's not beloved by the party, so he wants to keep all others out of the running. Trump probably has the opposite incentive: if we assume Trump won't have the necessary delegates to be the nominee, he probably has a better shot at emerging victorious if the anti-Trump vote is split. What will be wise for the rules committee to do depends on whether they think Cruz or Trump can beat the democratic nominee. If they think either one can, they should keep rule 40. If they think the party would be better served by Kasich or a dark horse (let's reconsider Rubio!) they should abandon rule 40. But to be clear: no candidate is entitled to keep the pet rules of the previous candidate. You want to influence the rules committee? Win!
[UPDATE/CORRECTION: A look at the actual 2012 rules shows that the measure in question is rule #40b (there are five articles in Rule 40). Additionally, I heard Karl Rove take the interesting position on Hugh Hewitt last week that Rule 40B doesn't prevent candidates other than those who meet the 8-state standard from receiving votes (in other words, it doesn't actually block other candidates). What 40B does is prevent anyone who hasn't achieved the 8-state threshold from receiving a nominating speech, with seconds and all that entails. The Romney people created this rule to prevent Ron Paul delegates from stealing the thunder of his convention.
Still a moot point because the rule doesn't exist, until and unless the rules committee carries it forward, but it's worth noting.]
4. Choosing anyone other than the frontrunner as the nominee would be to thwart the rule of the people. Again, nonsense. There is a way to become the GOP nominee. It's clear, it's fair, everyone knows what it is: win 1237 delegates prior to the convention. If you can't do that, that's your failure, we go to an open convention. Them's the rules, we all play by them. What would be unjust is saddling the people with a weak nominee out of a kind of pity -- "well, he came closest." There's a reason you have to meet a certain threshold to win.