I think we've all been expecting her graduation to happen, and I'm actually relieved that she finally announced it. As she said in ANN, she has "done everything there's to do as an idol".
You can't be an idol forever and this graduation definitely isn't a sad one. After all these years, she'll finally be able to focus on what she was made for: acting. I think Rena has a bright future ahead of her and it's time for her to move on. Also, it's better for her to leave while she's still relevant and at the top of her game.
Sure, it's going to feel weird watching SKE pvs without her but it's a great opportunity for younger members like Ryoha, Ami and Nao to seize. I hope they'll make the most of it.