I don't think any competitor will give you a dedicated gigabit to you for a reasonable price, especially if everyone suddently starts asking for one.
I pay for a Symmetrical gigabit connection, it’s 60 dollars a month. I record speed tests multiple times a day every day and have ever since I got the service last year. They’re growing gang busters too, my entire neighborhood is on Ziply (90% of household converted from what I understand)
Aside from them adjusting some things due to the rapid unexpected uptake, I have gotten full connection speeds for upload and download every day for over a year. Its been uptime of 99.999% (the adjustment period happened over 2 days and they only slowed service to 300/300 temporarily)
It can be done. It won’t be done by Comcast et. al.
Note that this doesn't means that there are exactly 0 users downloading in those moments. Usually there are multiple gigabits dedicated to a group of users, so that multiple users can navigate at 1Gbps without slowing down others, but not all the group at once. How much bandwidth is allocated to how many users can vary though, and some providers might allocate less total bandwidth to more users.
In practice this works out fine most of the time and most users won't notice slowdowns like you do, but if everyone started a speedtest at the same time you will notice it.
As long as the DMARC isn't saturated then you're only limited by the other lines also going to the DMARC and the collective activity of subscribers upstream of you between you and the central office. That's to say, you are right that it can be done but only insofar as other posters have indicated: last mile providers need to provision higher capacity lines between the DMARC and the central office.