The odds of Trump not securing the delegates are extremely low given he's poised to win in basically every contest heading into the nomination. It's also unlikely the Republican Party will take it from Trump given he's only 230~ away, despite Ted Cruz believing differently. They know Trump will go third-party and will very likely take many of his 10 million voters with him - though arguably they'd be okay with that because they'd also rather have Clinton, someone they know is not going to rock the boat.
Clinton-and-Sanders is still a contest at least. And Clinton imploding/getting indicted is far more likely than Trump given (as you say yourself), he's a walking outlandish comment. But he's also anti-establishment so the fact that Republicans in positions like Mike Pence, an anti-LGBT rights supporter of Bible-thumpin' Ted, means little to someone like Trump, who claims to be looking for support from the people and not the politicians.
I'm not sure how you're seeing that he's falling at a constant pace or she's increasing though. That list is fairly consistent around 7%-12%, with a handful of outliers putting her at ~20% and him ahead by ~5% or so. That's hardly indicative of much other than it's close in polling. Factor in debates during the general, possible implosions, and a mix of other factors, and there's not really much saying this is going to be a landslide yet.
"My call" is that it's too early to predict anything in an unbelievably unpredictable cycle. I do recall you making such statements about Trump early on, so good on you. I'd never have guessed he'd have made it this far or done this well for the nomination. But I don't claim to be an expert. :) Just one vote.
Well, allow me to put it this way:
There are two main possibilities. One, as you have been suggesting, is that Trump becomes a normal candidate, the race becomes a normal presidential election, blue states vote blue, red states vote red, and swing states split up their votes. Or maybe they all vote D like in 2012, but allowing for several republican incumbent senators to stay in office anyway. Or maybe even Trump wins because stuff happens. Nothing really happens and everything will be up for grabs again in four years.
The other possibility, which makes RNC memebrs stay awake at night, is very different. Trump, who is an "outlandish comment", runs as an "outlandish comment" because that's who he is. Hispanics and minorities feel that when he's going on tirades about "illegal immigration", he's actually saying "all non-whites", reinforced by tons of clips in which he's just saying "Mexicans" and a million quotes from the campaign. Minorities (and tons of women) show up to vote against Trump, and he does lose by 7-10 points as polls suggest right now. Blue states become bluer, all swing states become Lean D and Lean R states become swing. Not only that, but a ton of "innocent" republicans -reps, senators, governors, state legislators- go out on Trump's anti-coattails because all those people showing up to vote against Trump simply go straight D when filling up their ballots. And, as a bonus, most of those people decide that a party who has allowed someone like Trump to happen is clearly not for them and are turned off from ever voting Republican for a generation, setting in stone all those gains.
A Democrat's pipe-dream? A crazy dystopia? No, California.
And that fact that it already happened is what makes it all the more terrifying for the RNC.