Downsides -
It's not real legislation, no-one needs to act.
The deadline is long after all of the people in the current G7 will be dead. They won't even find out if anyone acted upon it.
2100 is an awful long way off, and continued placement of carbon into the atmosphere will make things worse.
There's a chance that some Tony Abbot-type of idiot gets voted into one of these 7 countries, and that nation repeals its involvement in the treaty. Hopefully not.
Upsides -
At least it's a law on the books, and agreed by 7 pretty darn big governments.
If anyone says Climate Change isn't an issue, it's made up/a hoax, it's not due to humans, or doesn't specifically focus on carbon, we can point to this law and get the doubters to answer why the G7 leaders all signed a treaty on it.
The current deadline is 2100, it sure won't be pushed back, but it may be brought forward. (who are we kidding... it will almost certainly be brought forward and be augmented by varying levels of draw-down details)
If there is a country that refuses to cooperate, the economic effect of G7 sanctions will be pretty brutal.
Once Tesla starts to advertise (if ever), they can mention this treaty as an additional reason to buy electric.