Travel time from Lyon to Paris

As long as the train goes directly to CDG from Lyon and you can afford to miss your flight home go for it. Yeah but keep an eye for potential strikes. This information is usually published online beforehand https://www.sncf.com/en/booking-itinerary/traffic-info/gi and you should get an email from the SNCF if your train has the potential to be cancelled beforehand. That was our experience in 2019.

I have done exactly this -- take a TGV from Lyon to Paris CDG the morning of my flight -- and made it, but I wouldn't recommend it and would never do it again. (The scheduling for this trip was not under my control/I would not have planned it this way.)

Two days before my departure, an SNCF strike was announced and it was not clear until very shortly before my train departure if it was actually going to depart or not. Had that train been canceled, there's physically no way I would have been able to transport myself to Paris CDG in time for my flight -- keep in mind the Lyon-Paris TGV is at at least twice as fast as driving (and probably 3x as fast once you figure in the time to take care of last-minute rental car facilities).

Fortunately, it was not canceled, but I know later TGVs were, and worrying about whether I was going to miss my flight due to circumstances out of my control was not a pleasant way to end my vacation.

In other words -- 5-6 hours is clearly sufficient time if your train is delayed, even by an hour or two, but if your train is outright canceled it may not be enough. If it is, any other form of transport is going to be much longer if on land (i.e. my point about driving above) or much more expensive if available by air (AF offers some flights from Lyon to CDG, but I can't imagine it'd be cheap to buy at the last minute.)

You should consider a late train night before and sleep at airport hotel or at very least back in Paris. It’s a LONG distance and a lot can go wrong.

It's not just strikes. I've been on delayed trains because someone forgot a package, requiring the whole train to be cleared and the bomb squad called in. Theft of electric cables for their precious metals during the night delays trains, too. Last summer during the heat wave some tracks were dangerous. My train two weeks ago was delayed while police were called to remove a couple of passengers. Each of these reasons has only a slight chance of happening, but over the years, I've experienced most of these. Since you have to pay for a hotel no matter where you are, just get back to Paris the night before, take the RER to the airport around 11:45.

If it were I, I would not take the chance of the train from Lyon making it to CDG on time, if leaving at 9:30 or 10:30 (that is NOT "plenty of time for the unknown"). There are much earlier trains, however. I might chance a 6:30 one but I hate getting up early and would not sleep well due to anxiety. I would go back to Paris and enjoy my one last night there--not an airport hotel. With a 3:45 departure, you could sleep in, enjoy a leisurely breakfast, take in one last sight near your lodging, etc. But, that's me.
(Is there any chance you could do an open jaw and fly back from Lyon?)
My husband and I just returned from a month in Europe. Our train from Barcelona to Girona was stopped because the train in front of ours had a mechanical problem--everyone on that train had to get off, get onto another mode of transport and then they had to do X to the train. Our 40 minute trip turned into a 3 hour one. Granted, this was in Spain, but could easily happen in France or anywhere.

I always travel on the day of my flight and have never had a problem. But if you're a worrier you're much better off travelling the evening before.

But you're not paying for a hotel, Simon. And you speak the language if the train is delayed.

When I fly to the US, I catch the plane at my local airport and change at CDG. But sometimes we combine it with a week in Paris before flying out. It's only a 3.5 hour train ride for me, but I wouldn't leave the same morning.

My kids had a 19:30 flight back to NY, so they caught a train from Marseille and were back in Paris in time for lunch.

Bets. I'd have to take a hotel room if I missed my flight rather than return back to the Loire Valley. Thing is, I always have travel insurance, so if some event should happen that means I miss my flight it's just admin. I've lived in Australia, I know what admin is.

In 30 years catching trains in France I have only had one serious delay, and even then I would still have had 4 hours to check-in

Many, many years ago when planning a trip with a travel agent he told us, “Always wake up in the city you are flying home from.” It made sense to me then and it stuck. Safe travels.

I'm curious what "the TMB" would be referring to?