Few things here:
Debt is not a factor in conference expansion (I'll assume if it threatens the viability of a program it would be, but that's not the case with GT). A lot of teams carry debt for various reasons (Michigan has about $15million less in debt than GT, new B1G member Washington has roughly $245 million in debt, Illinois has much more debt than GT at almost $300million) and if GT joins the B1G, our debt can be paid down quickly. Had GT joined back in 2012, we probably would have minimal debt on the books. I've written this before, GT has lost out on hundreds of millions the past decade since turning down the B1G.
When B1G invited GT to join in 2012, our average attendance was 43K+. In of 2023, our average home attendance is 34K. In CPJ's last year 2018, it was at 43K+. Clearly, the Collins years have turned away fans. If Key continues on the trajectory he's been on, more fans will turn out. If GT plays a B1G schedule, GT is probably at capacity for a lot of the games given the unique matchups and amount of B1G alums in Atlanta. That's a consideration for conferences. Also, conferences also understand these figures change year to year given the circumstances surrounding a team.
GT's brand is a LOT stronger than you think, and it's from multiple metrics:
#9 most recognized Logo
Ranking the most iconic and recognizable logos in CFB
ugawire.usatoday.com
#32 most recognized brand by top recruits (2022 survey)
Which college football programs are the best brands right now? Let's ask the top high school recruits.
www.si.com
#34 in Brand Value (2019 study, it's behind a Wall Street Journal paywall so I'll take the Reddit poster at their word)
This is an interesting one since it hits at the heart of why conferences expand in the first place. What additive value would a college bring to conference's media contract? By the metrics in the link above (and not including ND who is a football independent...and the "holy grail" for all conference expansion targets) GT is the 4th most valuable brand in the ACC. In terms of "true target" value for the B1G, GT is the 2nd most valuable school. Clemson, given their market and academics, is not a target for the B1G. The same can be said of VT. This puts us above UNC/UVA/Miami who have been historically linked to the B1G along with GT.
Choosing a team for expansion is a 50+ year decision. "Several really good seasons in FB and BB" (as you said) would not move the needle when a conference is looking at what the school would bring far into the future, not what's currently going on. If you look at the links I provided in my earlier post, those are the factors that school presidents, conferences, and media companies look at when forecasting the next 50+ years. A few good seasons, or a few bad seasons, doesn't change the the value of a school in the long run.