I will NEVER understand some people's mind when they rank Roger as #1 Srongest character of all time yet put Garp on lower end of top 10 AND some people even counted him out on top 10 current strongest characters just bcs he's a geezer lmaoo. For me, the current old, healthy Garp is still in the top 5.
Easily, you forget fact that Garp was bandaged when he faced Teach, who was much weaker than normal Younko, or even Admiral. So that's not even plot thing, that just fact which show us Garps power lvl in MF. While except that we knew only about fact that one man who was strong as Roger was called Whitebeard, not Garp. This lead some people, including myself, to wrong assumption that Garp was much weaker than Roger, but for now it seems that they was at the same lvl 38 years ago, or 12 years before Roger started his Jorney for One Piece. After that he faced Shiki with Sengoku, not along, while Roger defeated Shiki. I'm still not sure that Garp=Prime Roger, case the only one man was called
Strongest Man in the World, which made him above Garp, and that actually did no one other than Oda.
If Oda actually let Garp fight againts one of the Yonko now, do u think Oda will make him lose? he's defeated WB, Kaido and Mama's captain, and almost killed Roger many times. againts their mere subordinates? what yall think he gon do to them?
You already answered your own question, Oda just can't make Garp lose, that's why he will never let him face Younko or someone equal to them, like Akainu, in the battle, because today Garp is too old 78 years, and he is much weaker than he was before. Same thing was happened with Whitebeard, who was called as
Strongest Man, but in reality Akainu was on hil power lvl as well, due fact that Whitebeard was old, and sick.