If Welles had stayed with radio, he probably would have been faced with making a jump to television later on like all the other radio celebrities, anyway, since a life on the stage as an alternative could hardly have survived his legendary spending habits.
Interestingly, such a turn of events might have led to a very different outcome. Welles would have been incomparable when it came to pulling together exciting, frontier-breaking productions in the early days of live, black-and-white television drama. The invigorating diversity of building what amounts to a brand new stage play from the empty studio up combined with the relentless discipline of hard and fast broadcast dates/times together with miniscule budgets might have brought out the best in him. He so needed diversity, novelty, and chaos to harness his innermost creative energies while at the same time requiring an almost ironclad framework with which to shape them into a definite and finished form.
The years' delay in discovering his singular talent for the screen, however, might have led to a slightly mellower debut as opposed to the trademark style that his younger self had been allowed to explore at Hollywood's considerable expense. He might also have found the physical and technological limitations of early television extremely frustrating in relatively short order; so, a move to movies would probably have been a matter of time for him, anyway. Chances are that, as an actor at least, he would have turned in some pretty interesting movie work for the likes of Huston and possibly Ford before then, as well. Tough to know, really.
Although it's fun to "what-if" and imagine a longer and less bittersweet life for our hero, I fear the world might have never come to know the same rapture of experiencing the eclectic and edgy wonders that Welles succeeded in bringing to life for the delight of generations yet to come.