I wonder if JWs still hold onto this number as literal because it's one of the few things of C. T. Russell's that they still agree with. Then again - Russell said the 144K were those who'd inherit the "divine nature", while the "great crowd" was still a heavenly class, just not gifted with divinity.
Or - perhaps it's another power-play. If the number wasn't literal, than any old JW could be anointed, have God's spirit, and have a right to question, challenge, and generally disobey the Org without any meaningful sanction. Who has a right to tell you how to worship God when you're as much of a spiritual heavyweight as they are?