why do you not have a simple download version for older OSes? some/many others do it.
for example: version 10 in the App Store only works from version OS 20, then there is
version 9 for OS 17-19
version 8 for OS 15-16
version 7 for OS 10-14
version 6 for OS 9
version 5 for OS 7-8
version 4 for OS 3-6
and version 3 for OS 1-2
as download link on your site for download. (this is a example, version numbers are fantasy!)) the apple developer/store contract allow this and the benefit to your users who need old version is given. no additional effort for you (define: old version are out of support; old versions have not high traffic download rate, webspace is minimal…then, you are acting more serious than Adobe 
)