@Ostryga My opion is that if your DD's dad had died soon after she was born (and you had still been together at that point) then your DD would still have had some issues about it, but on the whole should have grown up fine with no major issues as long as she had you as a loving, caring and thoughtful (full of thought about your and your daughter's situation) mother.
However, if your DD's dad was to die now, while she knows and loves him but they are estranged, I do not believe that either of your lives would be easier. At the moment you are/should be (but I do think that I understand how hard that is to achieve, it is a very selfless act, but one that I think your DD needs) manipulating the answers, explanations, truth that you give to your dear 7 year old.
However, as she gets older she won't be so 'easily' deceived, and will therefore ask you far more probing questions, and as she reaches her teenage years she is going to want to cross question ask her dad in person and face to face - if that is possible - exactly why he 'left' her (as I think that that is how she will probably view his desertion). As she gets even older she will want even more answers from him, as her own life experiences into adulthood, and her own romantic relationships, will help her read between the lines and seek the real and/or complete truth.
If her father were to die while she is 7 years old, she will never have the chance to work through all her increasingly complex emotions. She will start to realise as she gets older that if you were being a good protective and supportive mum - which so far you give all indications of being - that she won't have got the full story (probably no-where near it) from you, so she will want, in fact need, to hear from her father his interpretations of events. If he agrees to see her and talk to her he will probably distort the truth as well, but not for such caring reasons. Your DD will probably eventually see through his evasions, and may decide that she wants to push things further, or she may decide that she has had enough and wants the estrangement to continue indefinitely.
If her dad was to die today, or somewhere in the near future, she would not have any possibility of learning the truth from/through him, and therefore not have the chance of being able to process it (maybe with some professional and knowingly unbiased help). I just think that it might be easier for you right now if he were to die soon, but when you have a distraught teenager, and troubled adult daughter, that may no longer be the case. I know it is very hard to make the correct decisions right now (or ever), I can look back now and see the wrong choices I made even though I made them with love. Obviously in reality you don't have any choice about when your ex dies, but I hope for your daughter's sake that it will be a long time in the future.
One other thought, it is possible that your ex will split up with his current partner, and come back into your DD's life, and stay in it.