I can generally see two motivations for your old PI to have you finish the project:
He benefits from a finished project and does not want to pay the price (time, risk, money) of training a successor.
He thinks that finishing the project is a good career move for you as you gain experience in wrapping up a project (and he can write that in his recommendation letter) or possibly a publication.
Something that is very important to consider here and in my experience often underestimated by students is this: It takes quite a while to familiarise yourself with a project so you can efficiently work on it. You never hit the ground running. The details strongly depend on the field and individual project, of course, but just to give an example, my rule of thumb for four-month projects is that the student has to be up to speed after three months and then most of the valuable work is done in the last month.
To estimate this for yourself, think about how much time and energy it would take you to re-do the work you already did on the project with your current knowledge (i.e., minus familiarising yourself).
Compare this to how much time you actually spent on the project.
Now, there may be some tedious experimental chores involved that simply cannot be made more efficient, but maybe you do not need to do any further of those or these can indeed be done by somebody else.
It may also be worth considering how much energy the remaining work will cost.
It may be very stressful for you, but it may also be some rather enjoyable, relaxing, and satisfying activity compared to everything else you are doing.
Finally, it may be worth noting that one thing your PI may have experienced and wants to avoid the following:
A valuable project that is nearly finished gets abandoned because the student quit or finished and was unwilling to do the few final pieces of work that can be easily done by them and only by them.
The reason for this is that the student does not recognise how much they could have benefited from finishing the project.
And training somebody else to finish the job is not worth the effort.
With all that in mind, I suggest to talk with the PI with all that mind.
I suggest to ensure to address the following points:
What work does still need to be done?
This is both about what he wants from you and what you think this requires.
What of this work needs to be done by you and what can be done by somebody else?
What’s the amount of time and energy you estimate to spend on this work?
How do you benefit from this?
Does this yield valuable experience, a better recommendation letter, a publication, etc.?
Mind that how direct you can be with all of this depends on your old PI and cultural factors.
