from ios 14.7.6 to ios 15.0

Very simply, you can only update an iPad to the most recent version of iOS/iPadOS that is supported by the device. Interim updates are not available. At the time of writing, for your iPad Air2, this is iPadOS 15.7.8.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed. 

System Software Updates are cumulative - and include all preceding feature and security updates.

Whether or not you are able to update any iPad to a newer version of iOS/iPadOS will be dictated by the iPad model - and the currently installed version of iOS. You can find this information here:

Settings > General > About

Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

32-bit devices (obsolete)

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi+Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi+Cellular)

64-bit devices

iPad Air1, iPad mini2 and iPad mini3 - iOS 12.5.7

iPad Air2, iPad mini4 - iPadOS 15.7.8

Newer models are all able to be updated to the current version, iPadOS 16.6

• iPad Pro (all models)
• iPad Air (3rd generation and later)
• iPad (5th generation and later)
• iPad mini (5th generation and later)

If your iPad is capable of update to a more recent version of iOS/iPadOS than is currently installed, these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that while an iPad may be capable of update, in some circumstances you may need to use a computer to perform the update. To do so, you will need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

as soon as the version of the IPSW is no longer signed it becomes useless

only signed ios versions can be installed, so an older youtube clip may be using ipsw that are still signed at the time

you can see what versions are signed for what devices here

iOS Signing Status - iDevice Central

No - it has absolutely nothing at all to do with planned obsolescence. It is all to do with ensuring that you install software that has known security vulnerabilities patched.

Digitally signed software can only be installed while its signature remains valid. Software with expired/revoked signatures cannot be installed.

You can install the most recent version of iPadOS that is supported by your iPad - which for the iPad Air2 is iPadOS 15.7.8 - and you a strongly advised to do so.

What, precisely, do you expect to achieve with iPadOS 15.0 - which has known Security vulnerabilities?