Well...Bush won two elections for Prez and his dad would've won a second if it weren't for Perot. Let's go back three decades. The 80's saw only republican presidents...the 90's were split with the Clinton years dominating...the 2000s were almost all republican years...and then Obama has won twice. Hmm...Slim is a funny word. I think the only way a Rep doesn't win this time around is if Trump runs third party. Hillary is an 80-year-old windbag, hippie, hootchie who's husband used cigars on fat chicks. She's still having to fend off weird Bernie Sanders. More than likely the Rep nominee will be Bush or Walker. I think either...I don't really care for either...will be your next Prez...unless Trump fires er' body.
The amount of Republican Presidents over the last 30 years really has very little to do with what's happening with today's electorate.
There are more African Americans voting now. There are more women voting now. There are more under 30s voting now. And, most importantly, there are way, way more Latinos voting now.
These are all blocks that the Republican party has in one way or another abandoned. Republicans should be much more successful among Latinos (and I think Bush can have success there) but their rhetoric around immigration drives Latino voters away.
Your logic as to why Clinton won't win is, I'm sorry to say, astoundingly bad and based mostly on your own bias. If Clinton doesn't win it will be because Sanders is simply more attractive to the base of the party (much like Obama was in '08) and because he may also attract some independents who like his mixture of progressive and socially libertarian policy. I think Sanders is certainly, right now, winning amongst under 30 voters. This is important because unlike elections you've mentioned, these guys are voting now. The internet has changed the voting pattern of young Americans.
I think you'll find Scott Walker will have a very hard time in the general because his record in Wisconsin simply isn't very good. I'm not even really sure why Republicans like him so much other than him fighting unions as he has exploded the deficit and recently even purposely defaulted on a debt payment. He's a Presidential candidate who may not even win his own state. He's also received a rather large amount of negative publicity already due to his slashing of the Wisconsin University system.
Bush I think would be an excellent candidate if he had any other last name. Although Clinton does carry the last name of a President that most Republicans despise, most Democrats and moderate voters are very high on Clinton now, 17 years removed from his scandal. The Bush brand is still very down with independent voters and even with some Republicans. Jeb will have to face many questions about his brother's Presidency, fair or not, and will likely have to say some hard things and distance himself from W.
When I look at the Republican pool right now, Jeb is the only one that I think actually has the resume that a Presidential candidate should have, has the ability to gain ground on the Latino vote (and win Florida, which would be huge), and carries the proper poise, aka "seems presidential." What remains to be seen if he can overcome the negative feelings towards George W. particularly since many of Jeb's policy ideas will be similar.
The truth about the Republican party is that it needs to split into two parties. You need the more establishment Republicans to shift a bit more to the center and disconnect themselves from the conservative wing. More and more there is a clear separation between the two different wings of the party and with the Conservatives pulling the Republicans further to the right it's allowing Democrats to grown their tent and take up more of the electorate. This is not likely to happen during this election cycle and maybe not even the next one, and if the status quo remains then the Republicans will have a hard time no matter who their candidate is and no matter who their opponent is.
