I think your guess is wrong...
Let me try to explain:
1) Updates to core service = satisfied existing customer, renewed subscritions. No change in new customers arrival rate.
2) No updates to core, but new cars / tracks = More new customers, new subscriptions. Existing customers have the choice to buy the new cars / tracks if they want to find races, so more income generated.
At the point iRacing has reached, they do not fear competition any more (if they ever did, they are americans after all
) - so all subscribers will renew unless they are burnt (eventually will stop sim racing for a while) out or out of cash. LFS is no threat (confidential sim, no advertisement or marketing), RF2 may be but... when???, NKP is confidential as well. So why invest massively to rush core updates that do not increase income?
In pure iracing marketing strategy, they will probably release core updates only when needed (LFS update, RF2 out or other threat). On the other hand they will continue to force cars / tracks down the throat of subscribers, because apparently they make a good profit (development vs income, marketing).
An interesting marketing option would be a full rental option, for a series (base + series content rented for the series duration only, expires at the end of a series but updated for the next selected series) along with the current pricing system.
Only my guess here... I'm open to counter argumentation