I think Mosconi's record has held because it's a hell of a lot harder to sustain a long run than people think. For a great player, getting to 100 is not so uncommon. A late friend of mine, George Mikula, did it nearly everyday in NYC. Getting to 200 is a lot less common. George would do that every week or so. Reaching 300 is phenominal. I never saw George get past about 275. And George was a superb straight pool technician, as well as a world class 3c player. 9-ball was child's play for George.
As for Mosconi, I saw him give two live exhibitions. In the first, he ran 57 and scratched when a ball kicked the cueball in. At his next turn to the table, he ran 93 and out. He turned to the audience and asked if we wished to see him run 150. Of course we all said, yes. So he did. Then he stopped and did trick shots.
The second exhibition, he ran 150 and out from the opening break. Then he asked if we wanted to see him run to 200. We did, and he did it. Then he ended his run, unfinished, and did trick shots. The table was a standard 4.5x9 Brunswich GC. I don't recall the pocket size, but I played on it a lot when I was in college and it was neither tight nor loose playing.
Anyone who questions Mosconi's genius never saw him play, and damn sure never played against him. I've heard many who knew him say he was tempermental, a prima donna and an irascible *****. I don't know if those claims are true or not, but I don't know anyone who saw him anywhere close to his prime who would challenge his greatness. And I never even heard of another player who could claim to beat him head to head ... excluding "Minnesota Fats" for obvious reasons.
As for a modern player besting Mosconi's run, I think John Schmit, Thorsten Holman or Danny Edwards might have a chance. Possibly Sigel in his prime, had he concentrated on straight pool. Reyes, as great as he is, would be a very unlikely choice. He has too many lapses and doesn't know straight pool well enough.