One of the presented options isn't like the others. As such the comparison isn't really fair, especially if you expect to run your business longer than 7 years.
If you buy more IPv4 space you will neither have to deal with CGNAT nor worry about traffic growth. Both of those benefits are easily worth the (short term) premium.
In the long term, buying more IPv4 blocks now is likely to be cheaper than running CGNAT for the foreseeable future.
To echo Owen, in general, the economics today still work out to make purchasing addresses more favorable than CGNAT.
If I assume the IPv4 purchased addresses will be useful for the next 15+ years they do make a ton of sense. Estimating the amount of traffic 5+ years from now is not something I have high confidence in. Making predictions is hard, especially about the future.
What kind of IPv4/IPv6 traffic ratio's should we expect 5-15 years from now? I assume there is no simple answer for this.
An ISP with mostly enterprise customer's would expect different assumptions from a mobile phone provider. This may be one of those times where every answer is correct, just not for everyone. The whole "one size fits some" kind of solution.
While I won't go into the costs as well, I've got actual work to do I must say my calculations of purchase ipv4 (@25USD/IP) vs CGNAT have always fallen significantly into the CGNAT camp. If you are doing a stand alone A10 or similar yes things would be different. If you are already buying suitable BNG's however the additional cost of MS-MPC cards (Juniper) or ISA2/ESA (Nokia) is likely to be far less than the stand alone option. While the BNG services cards are not cheap they do "just work" as a solution and a 10 or 20 to one ratio is easily achievable. Nokia 7750's with ESA "cards" are a massively scalable option.
We thought about it for a while at the ISP where I work, and went with Juniper MX960's w/MS-MPC-128G. Been working quite nice for us.
Initially, we went with smaller MX104 w/MS-MIC-16G to prove it out on our ~4,000 lower bandwidth DSL customers... when convinced, we then went all in with multiple MX960's w/MS-MPC-128G....now over 50,000 customers of dsl, cable modem and ftth
...all that behind about ~/21
I'll add that we already had the 960's for the 100gig mpls sp core we had built, so it was an investment only on the service module to do cgnat.