Infrastructure - management - Is moving from custom code base in place to Ansible worth it?
Solution 1:
Since your situation obviously isn't only about finding a good way forward from your current situation but also about politics in the workplace it doesn't look as though a purely technical argument will help you all the way.
Practically speaking I would say it's good that you already have a lot of "configuration as code", and especially if you have it version controlled in something like git, the main factor to do something about is readability and working to get rid of the bus factor:
It's bad for the company if important tasks can only be reliably performed by a single person: that's not real job security for the individual but it is a single point of failure for the company.
To introduce new tools, write down a business case to show your manager: what do you expect to gain, and how will this affect his bottom line? Create a proof of concept for some task you regularly do and show how a declarative configuration tool can make a difference in implementation or readability or in the sharing of skills between team members compared to imperative script languages.
And of course there's nothing that says that you have to use only one tool: shell scripts aren't necessarily obsoleted by tools like Ansible, but it sounds like in your company the way they're managed they may be slowing you down.
Solution 2:
I think it is worth to speak with and carefully listen to your senior, as you mentioned he's both experienced and a clever guy.
It is essential for your next steps to have a very good understanding of the underlying reasons, why things are the way they are now. Also ask about the challenges or weaknesses that should be solved from his point of view. Quite often there are underlying reasons behind a decision which are difficult to understand for people new to a company.
Depending on the information you received, you may need to answer yourself the question if this company is the right one for you, because the way you portrayed it here rises indeed some eyebrows. If you decide that the job is the right one for you, you should seek to help your senior by focusing on the issues that have been expressed. If not, simply update your CV and start searching new opportunities.
As others have pointed out in the comments, it is not the name of the specific tool that is decisive, but whether it is the most suitable tool for the specific circumstances and company. Given the specific situation, an evolution instead of a revolution could be the right answer.
An evolutionary step, for example, could be to continue to use the existing scripts, but to make them more convenient to work with, while expanding the range of people able to use the function the scripts provide. Maybe version control and the use of tools as Rundeck would be the adequate approach to step forward.
Frequently it helps to present short POCs in which you can show how a specific tool can solve a concrete task in a smarter way. Such an approach is often more effective to convince others than discussions or papers. You also should actively seek for some support within the company, because no matter which tool is chosen, it's how well it's accepted that matters most.
That all being said - personally I like Ansible too, you've already pointed out the perks. However, the way you have presented the situation, proposing Ansible could be mistaken internally as a (negative) revolutionary step or even as opposition.
The bottom line is to listen, and to act with some tact. Good luck!