It is possible to reduce data use, simply by using a small video window. With video full screen, I have found that Zoom uses about 1,000 kbps. If I reduced the size of the video window to around 512 by 240 pixels, the data reduced to 300 kbps. Minimizing the video to a "thumbnail" (200 by 112 pixels) reduced data to 220 kbps. Hiding the video reduced data to 120 kbps. This is consistent with Zoom's documentation: 1.2 Mbps for HD video, screen sharing with video thumbnail at 50-150kbps, and screen sharing only with no video thumbnail 50-75kbps.
During a formal presentation, video is really only needed at the start to introduction the speaker, and at the end during questions, if at all. In between there are usually slides to look at, so the video can be minimized, or hidden. I suggest participant adopt that way of viewing, and if possible, event organizers set this up as the default.
There are other ways you can improve video meetings, especially with careful preparation. The aim is to have the live video meeting as just one element, ideally a non-essential one. Much can be done with text based communications, which are less demanding of networks, to keep people in touch.
Zoom, and other video products, adjust to the bandwidth available, but then tries to use all that bandwidth. This makes them poor online citizens, like someone who fills their trolley with toilet paper, if you let them.
As there is likely to be a high demand for Internet access over the next few months, I suggest that providers of video conference products set defaults to use less bandwidth. At the very least they could be set so only a small video window appears by default. Also they could provide a low speed option which uses no more than 256 kbps, and this could be made the default setting. As an example, Zoom has a maximum bandwidth setting, but this is disabled by default. This should be enabled and set to no more than 256 kbps.