> Garp is old and while he's not sick, he has no feats to support he's as strong as he was at his prime.
There's nothing (as far as I'm aware) that suggests he's much weaker than his prime. For both Rayleigh and Whitebeard it was mentioned by themselves and/or other characters that they weren't in peak condition anymore. If Garp is so much weaker why hasn't anyone mentioned it yet?
Also Whitebeard being "The World's Strongest Man" is not a valid argument for Garp being much weaker. As the
Vivre Card databook states he obtained the title before the Great Age of Pirates which means Roger (WB's equal) was alive when he obtained the title.
Well Garp doesn't have a lot of feats the one notable feat he has suggests he's at least stronger than Akainu. Garp easily punched Marco away well Marco could block Akainu who was attempting to kill Luffy. Also Garp thought he could kill Akainu which there's no reason to doubt as nothing suggests he's vastly weaker than his prime.
> Blackbeard was intent on destroying Marineford right then and there despite Garp and Sengoku opposing him (+ the 3 Admirals in the vicinity). Shanks rolls in and challenges everyone who desires to keep fighting, and Blackbeard leaves, noting "We are not prepared to deal with you guys".
I also find this to be a bad argument. BB ran away from Akainu and even said "I'm not ready for that yet!". By this logic Akainu + 4 other admiral level charcters is less of a threat than Akainu by himself. It doesn't make any sense.
It's much more likely that BB was overconfident in himself (the same thing happened when he went up against Magellan who the easily defeated BB) because he obtained the Gura Gura no Mi only to realize he was way out of his league. This is supported by Whitebeard blantently saying that overconfidence and carelessness are BB's weaknesses.