thor: allfather giv me strength!

elf king: u understand boy.. u about to take the full force of the star.. it'll kill you..

thor: only if i die

elf king: yes... thats what kkk...killing you means .. (blur face)

he jump to the timeline back before 1970.
stays there until old. , until all time has past.
waaaayyyyy further which nobody knows how long.
old and retired, put on his 'old' quantum realm suits. jump to his 'past'. by doing this, he merged back to ENDGAME timeline, a bit earlier like few minutes earlier than he step oj that platform, behind the tree. take off his 'suit' and stays there. he MUST return, but he didnt. which explains he travel back to his 'past' from the 'old timeline'.

cap never return so it creates alternate timeline where there will be 2 caps in that timeline. definitely not the one we saw in winter soldier.

same timeline theory again..
now explained "the perfect condition" shield.
by staying in the same timeline, keeping/steal the shield would changed the present timeline.

that shield is a proof hes returning from another timeline.
its there for a reason.

quantum realm machine is a portal.
you jump into it, you can choose whichever timeline "in the past" , and must return on that platform to stay in the same timeline.

if cap re-appear to that machine, he came back from any point of "the past" to the present time.

if he didnt, that means he never return.

but he can re-appear on another location of this timeline "only if" that time was his past. in this case:

eg-
2023 is young cap present (T1)
1940 is young cap past (T2)
2099 if he stays this long and grow old, he can merge with (T1) on any location before he jump into the machine, which became his past. he chose 'the bench' to make it dramatic.

proof: cap+tony NY jump to 1970 without the machine. , doesnt matter if he havent been there as long they know the coordinate (tony's tech GPS)

get it?

portal open
time jump anywhere
return
portal close
whoever doesnt return will stuck forever

thats why u see nebula hv to reopen the portal, so thanos can go in. without that portal opened, the GPS tool become useless. pym particle also cannot help.

THAT quantum realm machine in this movie that make the time travel possible.

sam said something like "only it bothers me is the fact that we hv to live in a world without captain america"

old steve replied, "oh yes that reminds me of .. (open up briefcase with that untouched shield)"

i think, that timeline with the last dance "is" the life without captain america. maybe he let the frozed cap dies and get to keep the shield. hes the 'only' person at that time know the exact location of himself. so he stays low. no captain america.

he must hv been jump back to present timeline with the same quantum suit, take it off and wait on the bench. no other way.. definitely not the "stay in the same timeline" theory

you missed my point.
its bothers sam to live without cap "from that present onwards" . old cap is like "no worries, my timeline dont have cap too. see my shield. untouch"

its a fan theory "same timeline vs alternate timeline"

im more towards the alternate timeline theory "because" thats how MCU quantum realm machine works, u cannot be in the same timeline after u jump into it, once u jump, u have to return to the same portal, or else either trapped inside alternate time or have to jump back to the past timeline where the future never happen yet.