In its heyday, big conventions were not only big money makers for WT, conventions also raised the 'we are different/better than everyone else' hype. It plainly demonstrated member growth year to year and it as was only an annual event, it gave JWs something different from the usual routine to look forward to, as difficult as that may now be for most of us here to believe. They were also typically mentioned in newspapers and local TV news, but less so now.
In addition, it was a method for R&F witnesses to demonstrate loyalty by attending an event, sometimes a few hundred miles/kms away, for up to 8 consecutive days, from 9am to 9pm.
Now that growth is minimal, stalled or declining, the income received will be also declining and it will be increasingly more difficult to 'visibly' fill big venues.
As more JWs are waking-up to TTATT, it will also be more difficult to disguise JWs that were commonly recognized at every convention, are no longer in attendance. Disguising this is more easily accomplished in smaller venues, by simply reassigning which congregations are to meet together whereas with larger venues, it is generally the same districts and congregations year after year.
As Assembly and KHs are already owned by WT, there would be little additional expense incurred in using those facilities as RC venues. Although contributions maybe less, a higher percentage would be retained.
Since JWs are already expected to take time away from work to attend a 3 day event, I expect some RCs will not only be held from Friday to Sunday, but could also be held during weekdays such as Monday to Wednesday.