My point: it doesn't matter how long they give us to switch - most people won't do it until they are absolutely forced.
This reminds me of Microsoft back in the day. Windows was (is?) crap, unreliable, and without the kindness of technical friends and relatives willing to work on someone's Windows issues for free, Microsoft may not have ever made it past DOS. I got sick of working on everyone's Windows problems so got rid of all my Windows machines and just told people "sorry, I don't have any Windows machines anymore and can't help you". And it was the truth - after about a year I really didn't know how to help them anymore! It's great!
Maybe it's time for technical people to stop working for free for Microsoft and Google. If their stuff is crap, let them sort it out with their wonderful customer service that you now have to pay for.
People need to assess the situation, evaluate existing solutions (including paying for the service), what needs to be done, etc. They're not doing it as their job, so do not necessarily know the current market offers.
Then they need to allocate time and resources to do the transition. Time, when the transition is the least disruptive, and the resources are usually availability if an in-house specialist. I know plenty of companies where upgrading a software take more than a year, with all their budgets, IT departments and external contractors. A typical G Suite user won't have all that expertise readily available.
That means that the transition will often require an IT enthusiast working in their free time. It's reasonable to assume that they will have at least one vacation per year to do that. So if the transition is ≥ 12 month, everyone and their mom should be able to do it.
If the transition is 3-5 months, I can see how it can be a problem even for commercial users with a dedicated task force.