I tend to lean to your way of thinking. Trump just had too much momentum and his win in Indiana put him up 25-4 over Cruz in terms of primaries won to date and 25-1 over Kasich. Other than Maine, a caucus state, Cruz won nothing east of the Mississippi and Kasich only had his home state of Ohio.
At that point, the GOP realized they had no choice but to cast their lot with Trump and make the best of it. So they shut down the money spigot and pulled the plug on Cruz and Kasich.
It was just announced a few minutes ago that Trump will not be self-funding his campaign for the general but will take GOP money.
The Republican Party are now going all-in with Trump.
By being able to self-fund, Trump is not vulnerable to being betrayed by having his funding from big-money donors suddenly shut off in the middle of the campaign.
He can take GOP money, but can go the distance without it. This allows him to give the finger to any or all of the big-money donors, whenever he wants. He can use their money, and keep his own money in reserve.