The main issue is that this is not a conscious decision that google asks you to make. They offer a free mail service and then turn around and harvest your personal mail to increase their revenue, without most consumers realizing it.
They could've been happy with your lock-in to the google ecosystem and to google search, but they wanted to more data and systematically scoured all transactions from users' inboxes for additional data.
Like with physical mail, the digital mail has an envelope and a message. Gmail offers to handle your envelopes and assists you in viewing the message, but then went a step further and data mined all info from the messages.
In the physical world, if a company handles my inbox, it would be forbidden to open the envelope and read the message without a proper consent. The company would just open the inbox, tell me there are two letters from the bank and one letter from the customer, without reading the actual message.