Redefinition of \maketitle is ignored

Question

I would like to change some elements of my title page, but without completely creating my own titlepage environment (as e.g. demonstrated here), because I am using commands like \extratitle and \uppertitleback and I'd rather not recreate them all.

I am using a KOMA-class and luckily, this post not only explains how to redefine \maketitle in such a case, but also includes the original definition of it, so it is easy for me to adapt.

But for some reason, my new definition is ignored when \maketitle is called, and the default one is used instead. It gets even stranger however, because when I call \makeatletter\meaning\@maketitle\makeatother (which should show me the current definition of maketitle according to this post), it produces the new definition, both before and after I call \maketitle. What's going on here?

Also, when I create an entirely new command that way, e.g. \makecustomtitle, the compiler says it doesn't know it: Undefined control sequence.

MWE:

\documentclass{scrbook}
\makeatletter
\renewcommand{@maketitle}{%
% \newcommand{@makecustomtitle}{%
%   \global@topnum=\z@
%   \setparsizes{\z@}{\z@}{\z@@plus 1fil}\par@updaterelative
%   \ifx@titlehead@empty \else
%     \begin{minipage}[t]{\textwidth}
%       \usekomafont{titlehead}{@titlehead\par}%
%     \end{minipage}\par
%   \fi
%   \null
%   \vskip 2em%
% %  \begin{center}%
%     \ifx@subject@empty \else
%       {\usekomafont{subject}{@subject \par}}%
%       \vskip 1.5em
%     \fi
%     {\ifx@subtitle@empty\else\usekomafont{subtitle}@subtitle\par\fi}%
%     \vskip .5em
%     {\usekomafont{title}{\huge @title \par}}%
%     \vskip 1em
%     {%
%       \usekomafont{author}{%
%         \lineskip .5em%
%         \begin{tabular}[t]{@{}l}
%           @author
%         \end{tabular}\par
%       }%
%     }%
%     \vskip 1em%
%     {\usekomafont{date}{@date \par}}%
%     \vskip \z@ @plus 1em
%     {\usekomafont{publishers}{@publishers \par}}%
%     \ifx@dedication@empty \else
%       \vskip 2em
%       {\usekomafont{dedication}{@dedication \par}}%
%     \fi
% %  \end{center}%
%   \par
%   \vskip 2em
}%
\makeatother
\begin{document}
\subtitle{This is a subtitle}
\title{This is a title}
\author{Author Name}
\date{\today}
\makeatletter\meaning@maketitle\makeatother
\maketitle
\makeatletter\meaning@maketitle\makeatother
% \makecustomtitle
\end{document}