Too late to debate this now, but there are more than sufficient ports in Europe that can accomodate 6000+ pax ships, so that is not an issue (and, lets face it, the ship is the destination not the ports). The new millenium cruiser does not really care about the itinerary - they want to be entertained and fed and watered, which is where the profit is.
And, pro rata Covid cases are the same, regardless of the size of the ship.
Feel free to comment on roi with regard to capex and anticipated depreciation at your leisure.