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 am 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.