Sure... and as I've posted before, Flightline 730 from Frimley to LHR T5 is £3 and a single 40 minute bus journey. From bus to gate is a maximum of 30 minutes via the FastTrack lane

Or a bit longer if you want to use the lounge...
Amusingly, Google Map's quickest route to St P is to get the same bus to LHR, then the Lizzie, for a further hour or so journey.
St Pancras is good for some, but not so good for many; of course, anyone on the EMR route is quids in...
Although, due to it being an hourly service that could add a significant amount of extra time to the journey. For example, if the fight is of a time when you need to catch the bus the hour beforehand.
That's before we consider that there's a 35 minute cut off rule, where one has to enter the security at Heathrow with at least that much time otherwise your check in is automatically cancelled.
That means that, even being truly optimistic (that the bus isn't even 30 seconds late) it's at least 46 minutes from departure time.
It also depends on where exactly you are traveling, much further out on that bus route and the maths changes.
Also, rail links to Heathrow from the Aldershot Urban Area and most places in the SWR area are not great. Ideally it would have the Southern Approach to Heathrow, which would give Farnborough Main a more reliable more frequent service (even more ideally it would be a stopping service from Basingstoke, as the time penalty would be limited and then only for those traveling from Basingstoke).
Likewise, the slog across London slows things down to get St Pancras.
The current circa 90 minute (Farnborough Main to St Pancras) could be reduced to circa 70 minutes with Crossrail 2.
Of course Heathrow is far closer than St Pancras and so would always be fairly quick to get to.
Having said that, my aviation geek friend has said that they much prefer using Eurostar to get to Paris than flying and so it's likely that even an extra hour isn't gong to dent the choice made by many.
Especially given that Air France operates 6 services a day whilst Eurostar operate 16, and so you're much more likely to be able match up arrival times to suit your requirements by going by train. Especially if you are considering what happens if a service is cancelled (I.e. taking the option before the one which would be just in time, so you're not late even if a fight is cancelled and you still manage to get a space on the next fight - or train if that's what you're using for travel).