Goldberg is one step too far in the "returns" stakes for WWE to risk.
They have got Warrior now on the hook, Hogan is probably on his way. That's 2 major Mania 30 spots gone for their current roster. If Rock and Taker are back, much less Austin then you are running serious risks of Talent deciding it isn't worth it...particularly guys like Punk who has been waiting for his chance to headline Mania proper.
Vince/Trips et al have a real dilemma at Mania 30 - cram it with every legend they can for one final hurrah or as they did at 10 and 20 focus on the new generation talent with cameos by older stars. 10 and 20 worked well, so changing it up for 30 is messing with the cycle that has worked well for 30 years.
Don't get the cycle? Look at this:-
WM1 - New main eventers established with Celebrity involvement in a grudge feud (Hogan, T, Piper)
WM5 - Dream Match - Hogan v Savage
WM8 Hogan vs Sid - Veteran Talent vs Young Star intended to replace him
WM8 Savage vs Flair - Veteran beats New Main Eventer who hasn't worked out as planned.
WM 10 - Launch of new main eventers for next few years with Bret Hart and Shawn Michaels taking prominent roles.
WM11 - New main eventers in grudge feud with celebrity involvment (Diesel v Shawn with Pam Anderson & LT vs Bam Bam)
WM15 - Dream Match - Rock v Austin
WM18 Hogan v Rock - Veteran vs Young Star intended to replace him.
WM18 Trips v Jericho - as with Savage/Flair.
WM20 - As with 10 but Eddie and Benoit (how it turned out is immaterial)
WM21 - New Main Eventers given title in Grudge feuds (Cena and Batista) and Celebrity involvment (Akebono)
WM25 - Dream Match - Shawn v Taker & Trips v Orton
WM28 - Rock vs Cena - Veteran vs Young Star intended to replace him
WM28 - Jericho v Punk - Veteran puts over young star.
WM28 - Sheamus v Bryan - Champion who hadn't worked out replaced as with WM8 and 18.
The pattern is there and it's probably why WWE has made Mania stick for so many years. You know at the start of the cycle you get the new guys beating the old, building up to a crecendo in the middle of the decade, when the focus shifts to finding the next guys to move into the positions. By the 8th year someone will have failed before they finally settle on someone for the turn of the new decade.
Where WWE has struggled this time is settling on someone for the next cycle...it may be Punk, Bryan, they may stick with Cena they may suddenly elevate someone with the Rumble... but to flood the card with veterans this time would be a risky strategy especially like Goldberg who'se rep is the shits with most of the guys.
How would the younger guys respect him, knowing Chris beat him? They wouldn't so you'd end up with another fight and someone getting hurt. If he had a year and agreed, like Lesnar to drop his first 2 or 3 matches then he might be able to put someone like Barrett over effectively but he won't do that - he'd want the title and it would be as bad as last time or they'd force the Ryback match and it be awful.
Warrior sure, give him a 3 minute squash over Big Show or have Barrett beat him... Hogan, hmm if we must have one more match have him put Cena over so we at least get one "dream match" out the way. But Goldberg is one too far... he would serve no purpose, bring no benefit nor create anything memorable or help those talents get over... if a Barrett or Ziggler beat him "well he didn't wrestle for 10 years, what do you expect?" would be all it would get from most fans.