Out of curiousity I searched for an ABAP tag in this forum and found this question. I think I'm in the position to comment on your three items.
i) this is not a correct view on things as they are. What is true is
that ABAP is used exclusively for programming SAP systems, but this
is not happening exclisively at SAP. SAP systems can be found in a
rather large number of large and huge companies in almost any
industry sector and there is a whole ecosystem of companies which
offer SAP know how (including ABAP programming) to them in each of
these sectors. That is, if 'outside world' means outside of SAP, your
statement i) is simply incorrect.
In addition, writing software has several aspects to it, out of which
the programming language is probably not even the
most important one. If it comes to writing software which is to be
used by many large companies all over the world in a productive
environment then questions like business know how, deployment,
backward compatibility, maintainability, overall architecture,
internationalization, ... (the list can be conitnued for quite a while) become much
more important than the programming language you might be using.
Depending on your job you will be able or even need to learn a lot of
things not related to the programming language at SAP (or any other
company creating software for that kind of customers)
ii) this depends on you and to some extent on the position you would
be holding. I think there are (few) jobs at SAP where keeping contact
with the non-ABAP world will be solely based on your own engagement.
There are also jobs at SAP where you are programming in JAVA, C++,
C#. The SAP kernel is written in C++, to my knowledge. And, of
course, SAP is interested in keeping their own technology stack as
well as their employess up to date. Every company of that size will,
or will soon become irrelevant if they don't. As far as I'm concerned, ii) is a
iii) ABAP OO is quite OO, if you ask me. The code base you will be
working on may not be in ABAP OO, though, but plain old ABAP. In ABAP
OO you will possibly miss some concepts you like and may find
concepts which look strange, similarly you may find that there are
interesting details to the language as well. The same happens if you
are used to writing programs in Java and then start to write programs
As for the comments after one of the other replies: I do think you can 'go back to normal programming after it', assuming you are working consciously in each environment you are using and assuming you are open minded. I know some programmers who have learned to do things in a certain way and have a hard time to do them differntly after some years. In that case you would have a hard time to leave the ABAP world. This is not due to the programming language as such but to the infrastructure you may be getting used to.
Please note that I do not suggest that you apply for that job, neither do I advise against it. But the concerns you expressed are, from what I know, not adequate as a basis for a decision.