The short answer: ODB needs to get new people to at least sample the station (i.e., Trial, aka new cume) to build up the total cume (including replacing tradional oldies partisans who deserted the station when they heard Tears for Fears).
The fleshed-out explanation below goes beyond your specific "Cume=Trial?" question, but I feel I should provide full context:
Take a new product introduction, say, a new brand of toothpaste. When a consumer buys it for the first time this is considered a "trial" purchase. So if, say, 20 million people have bought the product within 12 months of rollout, its trial is 20 million. Assuming the product generates sufficient trial, the next step in determining longer-term potential is to assess whether enough of these triers were sufficiently satisfied (or otherwise incented) to come back and buy again ("repeat"). In consumer panel data, the classic analysis for judging how a new product is doing is in fact called "Trial & Repeat." There have been new products -- e.g., novelty items -- that have great sales initially, and it looks like major success, but then they quickly start tanking. The Trial & Repeat would've shown the underlying dynamic, namely that you're getting lots of people to try the item, but that few are coming back and buying again.
For a brand new radio station, its initial cume would be the "trial," although you'd probably be looking at a shorter period than a year. The same for a format change, although it's muddied a bit by retained cume (for an extreme format switch, or for a station whose cume really balloons up after the switch, the analogy is cleaner). TSL is probably the closest analogy for repeat. A brand new consumer product (as opposed to a line extension) generally needs at minimum 25-30% repeat to have a chance at long-term success. 40% is considered a solid showing. Most new products never make it to these levels, and they fail.
For an established format, the analogy would be to an established toothpaste like Crest. Cume=the number of people buying at least once in a given period (usually called penetration or reach in marketing lingo), while TSL=the amount of Crest the average Crest buyer purchases over that time period (often called "buying rate," which tends to be high when product loyalty is high). You can see that you need both to sustain a product, although without at least a reasonable amount of penetration (cume), no amount of buying rate (TSL) will be enough.