Technology marches inexorably forward. These algorithms already exist in unbroken form. What the government is doing is mandating that Apple and Google make user's data less secure. The government will claim that any such backdoor will only be openable by them and only in cases of great need (read terrorism).
What will happen is that non-governmental organizations will find these keys and get the same ability. What will happen is that the government will overreach, as they have done before time and time again.
This isn't about stopping terrorism, this is about keeping the current ability to spy on US Citizens. The internet was the single greatest intelligence boon in history and the government is concerned about loosing spying capacity.
Unsophisticated terrorists would gain little protections from at-rest and over-the-wire encryption. It's really quite easy to screw up and be found in the dragnet. Sophisticated actors would be using additional software anyway. You know, the kind that already exists.
The libraries are free and public. The algorithms publicly published. You can't put the cat back in the bag.
The government is trying to legislate particular usages of math. They are literally saying that Apple and Google shouldn't be using certain sequences of numbers. This isn't an exaggeration or hyperbole since everything is encodable in numbers and obviously some communication is illegal.
This is literally illegal equations, and it is so very far down a slippery slope that every citizen should be concerned.
To answer your question, the government will still not be able to decrypt many communications, even with this backdoor. These technologies protect casual users from data theft, but putting in backdoors makes everybody less safe.