Most of these questions are addressed in other Bay-verse media; comic books, novels, ect. If you are like me

and can't attend the movie theatre for medical reasons, this stuff is a life saver!

Anyway, I wondered about this too. So here are my conclusions. Feel free to point out logical errors.
All thirteen primes set out across the galaxy to find suitable worlds for star harvesters. I got the impression that they had found several and repowered the Allspark more than once. The Fallen

just got lazy or thought a life filled planet would work better so they had to stop him. But it is quite possible the Allspark was brim full of Energon when the Primes sacrificed themselves.
Optimus clearly was worried about the amount of Energon Megatron would have access to if he captured the Allspark, that’s why he sent it off world in the first place, so it must have been pretty charged when it left Cybertron.
According to the Fallen it was no coincidence the Allspark headed straight for Earth after Prime sent it into space. It was in the Allspark's design/programming to seek out star harvesters if left adrift. This is why the Fallen ordered Megatron to go after the Allspark himself instead of continuing to battle Prime and the Autobots. Send out the fastest, strongest warrior to capture two birds with one stone.
The Allspark had easily fueled an entire planet of billions of sentient machines and their infrastructure for millennia, giving birth to untold numbers of Cybertronians before it came to Earth. Running a few human machines for a few decades and reviving a handful of Cybertronians wouldn’t have been too great a task.
In the novels Ironhide explains to Staff Sergeant Epps that Energon is naturally occurring. There is probably some on Earth even that could be mined. But they can make an artificial Energon substitute that can sustain them just as well as the real thing and is much more stable. Jetfire didn’t have the resources to make his own or to mine and purify the naturally occurring sources. With the aid of the humans infrastructure the Prime team did.
But to harvest and purify and transport the amount needed to sustain a healthy Cybertronian planet requires the Allspark and a star harvester. To rebuild a dead planet requires even more. This might be gotten around with
Sentinal Prime’s space bridges. The ability to instantly transport smaller amounts of refined Energon from many slave worlds could have sustained a smaller population.In fact the only single life function that the Allspark is statedas being essential for is the creation of new Cybertronian life, ie reproduction. That is what Megatron meant when he asked Prime weather their race wasn’t worth one human life in ROTF. Not that every Cybertronian would die if he didn’t mulch Sam’s brain

to get at the dissipated Allspark energy, but that no new ones could be born

.
On a side note this could explain why the Autobots, especially Prime, put up with so much from humanity

. Realizing that their own race has ruined its chances of future reproduction they have adopted the humans as surrogate children. Optimus Prime displays a hyper developed Pappa Wolf instinct in every incarnation but especially in the Bay verse, as seen in his interactions with Sam and the human soldiers.