Nothing is certain in life, Inyokern. The scenario of what happens if Alexi wins isn't guaranteed, but it is very likely Brady will win, Alexi would get indicted, and Brady would name a conservative replacement.
Alternatively, the "reasons" we're given to vote for Kirk is that he will be a vote for a Republican Majority Leader and Kirk will cast the key vote to put the Republicans in charge of all the committees, then the Republican-controlled Senate can promote extended tax cuts and control the agenda and everything will be all rosy.
This scenario has even more disqualifiers:
1) It's not certain the makeup of the Senate would be split exactly 50-50, thus making Kirk's election the instrumental 51st vote on who control's the Senate.
2) It's not certain Kirk would vote for a Republican Majority Leader. He has made no such pledge to support whoever the Republicans nominate. And given that Kirk is an idealogical Jeffords clone, it's entirely possible the Dems could convince him to vote for their guy is the event you did you have a 50-50 senate.
3) It's not certain Kirk would vote YES on all tax cut bills in the Senate. Kirk has done so previously as a House member, but he's also shown a willingness to compromise or flip-flop on ANY issue for political expediancy. For example, he always voted with the GOP on Iraq until the surge vote when he sold out for political expediency. If "the polls" showed the American public supported the RATs on a tax bill, and the mainstream media was heavily promoting the RAT position, it's entirely possible Kirk could switch to the RATs on that bill.
4) It's not certain such legislation would reach the Senate floor even IF Republicans controlled the Senate on paper. Thanks to having lots of RINOs like Kirk, they might have a majority on paper only and the RATs could be exercising defacto control of the Senate, filibustering legislation and locking it in committee, preventing it from every coming to a floor vote, so Kirk wouldn't be weighing in on the legislation.
5) In the event such bills ARE passed, it's not certain they would become law. Obama would still be President, he would be likely to veto GOP-written legislation. It's almost guaranteed the Republicans would have 2/3rds membership in both bodies, thus they would have a hard time overriding a presidential veto without a large number of Democrat crossover support.
So if you want to look at things objectively, it's a lot easier for scenario #1 than scenario #2 where we have a good conservative Republican Senate passing legislation thanks to Mark Kirk.
Now you are just being silly. He has been a Republican congressman for 5 terms from a district that is rather liberal. If he were going to change parties, he would have done it already.