I mean Trump is a pathological liar but he’s also got an ego the size of Texas. Signing the pledge would be, if only symbolically, ceding his free agency. He’d probably say something like:

What’s there to sign? I am the nominee. The rest of these knuckle heads just have a hard time admitting it. And frankly speaking I’m the GOP when you think about it. The fact the RNC is too scared to put their money on me from the get go is a sign of weakness and this is why we’re having a hard time winning elections.