Grandpa Longneck is a supporting character in the Land Before Time franchise. As his name suggests, he is Littlefoot's grandfather. In a lot of ways, Grandpa Longneck is kind of the emotional core of the franchise. He is a caring, compassionate and sturdy (both physically and mentally), who characters come to for advice and always seems to have a level-head (which comes in handy when you have to rescue precocious young dinosaurs all of the time).
I cannot say enough good things about Kenneth Mars as Grandpa. I don't know what exactly it is, but Kenneth's voice is just so iconic to the role. It's warm and caring, elderly and premeditated yet also has the depth that adds size and power. When Grandpa Longneck spoke, dinosaurs (and I) listened. Every line of dialogue he spoke was strong and calm.
After Kenneth's unfortunate passing, it was quite the headscratcher as to who they would cast. Would they simply remove the character? Would they limit his screentime and not give him any dialogue? Would they recast? If so, who would they choose? Common ideas include VAs like Jim Cummings, who had doubled for Mars as King Triton in The Little Mermaid: Ariel's Beginning. Or, you know, other deep-voiced VAs. I can say with 100% honesty that I would've never seen Barry Bostwick coming. Barry's voice isn't very similar to Kenneth's, but I find that it works anyway. They seemed to go for Bostwick doing his own thing rather than trying to emulate Kenneth. Barry's voice is lighter, but has a similar "elderly"-tone to it.
In case it wasn't obvious, Kenneth is getting my vote. On it's own, Bostwick's voice is fine, I just find that Kenneth is a hard act to follow.