What kind of math are you using?
Ottawa's average attendance in 2017-18 was 15,829 (ESPN), which included the outdoor game at Landsdowne (33,959) Park, and one game in Sweden ......... so the average attendance at CTC was closer to 15,375.
So a decline of (close to) 50% , according to you, would have been about an average attendance of 7,700.
According to ESPN the average attendance was 14,553, a drop of about only 5.5%.
Sure, the fans in general have not been happy with the loss of many star players that they've seen drafted, developed and played for the Senators for many years .......... and there are some, like yourself who are probably extremely upset over the direction the franchise has taken.
But it's not the entire fanbase that feels as bad you do, and there will probably be a similar attendance average in the coming season, but it's eventually going to improve when the teams' play improves and they show signs of competing for the playoffs in a couple of seasons.