Earth Prime-Time already has its own version of Barry Allen, who is better known in the comic book universe as the Flash. As far as being seen in the main Toobworld, Barry Allen first appeared in 1990 when a chemical explosion in his lab transformed him into the Scarlet Speedster.
The Trueniverse audience only saw him in action that one season, but Life goes on even after cancellation. I see no reason why Barry Allen should not be still alive in Toobworld.
But as for still being the Flash? If the Flash still existed as was capable of such high speeds, why isn't he being mentioned as foiling crimes in Central City still, or somewhere in the country?
Here's what I think happened - the high metabolic rate of his body eventually drained off all of the ability for Barry Allen to travel at high speeds. (Such a situation could also have gone unchecked until it finally killed him, but why do superhero stories always have to have such morbid endings nowadays?) I prefer to think that once he was bereft of his powers as the Flash, Barry Allen returned to his civilian life as a forensic scientist for the Central City police department.
He may even have opted to cash in on his former popularity by writing his autobiography, selling the rights to his story to the televersion of DC Comics, and making some coin off the merchandizing rights to his logo and costume.
Something along those lines must have happened. On 'The Big Bang Theory', Leonard, Sheldon, Howard, and Raj all owned Flash costumes. Sheldon also owned at least one T-shirt with the Flash logo, as did Screech in 'Saved By The Bell'. And in the "Lion's Den" episode of 'The Outer Limits', Brae referrred to Morris as the Flash. Also on TBBT, the Flash comic book can sometimes be seen in Stuart's comic book store.
In fact, in the pilot episode of 'The Middleman', Wendy Watson told her new boss that "The Flash" was her favorite comic book. The Middleman responded, "Barry Allen or Wally West?" From this we could make the conjecture that DC Comics created Wally West as a fictional successor to Barry Allen, or perhaps Wally West also exists in Earth Prime-Time but we just haven't seen him yet. (I prefer to think he was fictional; otherwise DC would have had to deal with Wally to secure the rights to his story.)
Getting back to 'Arrow', it's not a total write-off for the series as far as Earth Prime-Time is concerned. As was the case with 'Heroes', 'Stargate SG-1', 'The Dead Zone' and even 'Alias Smith And Jones' before it, the preceding episodes of 'Arrow' before Barry Allen makes his appearance can still be considered part of the main Toobworld. So that would mean thirty episodes of 'Arrow' do exist in Earth Prime-Time, and that's not such a bad legacy, is it?
But from "The Scientist" onwards, the series is following the Oliver Queen of an alternate TV dimension.
Perhaps even the same dimension currently occupied by 'Marvel Agents Of S.H.I.E.L.D'.....?