I had some good years with the alcoholic I married before the disease (and mental illness) took him away, and effectively killed the person I married. We were together for 25 years. Would I have given up those early years, if I had known what was coming? I think that yes, I would have chosen differently if I could have known what would happen. He is literally not the same person that I used to know, and a lot of damage has been done to me.
However, there are people on this board who have stuck with their alcoholic spouse long term and don't regret doing so. Your mileage may vary.
Two thoughts:
1. BIRTH CONTROL, BIRTH CONTROL, BIRTH CONTROL. You absolutely do not want to have a child with this man. You can make choices about whether you will run the risk of a relationship with an alcoholic, a child cannot. And if you've got a child, you will not be able to get entirely free of him if/when he deteriorates into someone you don't want to be with.
2. Don't let your decisions be swayed by "well, I've invested x years with this person so I don't want to/can't give up now ...". Don't let the length of the relationship influence your decision about whether you want to put more of your life into it.