The conventions , including the international ones , are huge money spinners for the Org and really motivational events. I just don't see them scrapping them ( as stadium events ) any time soon - they make money from double donations ( congregations paying in advance for the bookings and then JWs donating in person ) , commission on hotels , kickbacks from cities etc etc - its too much of a cash cow to let go. I think basically they have to plan 12/18 months ahead to book locations and they decided that 2021 was too uncertain. I see lots of businesses here scrapping major events as well in 2021 , or moving them tentatively to Sept / Oct "subject to change".
I think the Society will be back with in 2022 with a "special" convention program with some big international events - just to keep people interested.