I now hardly ever pray Eucharistic Prayer I, but I have reasons other than just "well, I'm getting older now, and I've been in the trenches a while, and I'm just chilling out a little bit"
One more comment by way of introduction: put yourself in the shoes of someone who wants to check out Catholicism for the first time. We HAVE to have something to give guests at Mass which allows them to follow along, from beginning to end, that tells them when to sit and stand, what the Mass parts are, what the Eucharistic Prayer is, what our policy on receiving Communion is, etc. I have come to believe this is really important. I have been donating the printing costs for these at my parishes because I feel it is super important.
So, in order to have something to give to parishioners and guests alike that will allow them to follow along in this fashioin, the Eucharistic Prayer cannot be a week to week choice - one has to be chosen that will be used for as long as those particular booklets are going to be passed out to people coming to Mass.
All that being said, there are a couple of very intentional reasons I don't pray Eucharistic Prayer I much
1) There are a TON of theological concepts in Eucharistic Prayer I. Each sentence could be prayed over for hours! In fact, when I did my retreat to prepare for my priestly ordination, I prayed over various phrases and words from Eucharistic Prayer I over the course of 5 days.
Most people do not have 5 days for a retreat on a particular Eucharistic Prayer, but I do think the parish should have opportunities to intentionally reflect on the particular words of a Eucharistic Prayer through a day of reflection and/or homilies, and that particular prayer should be used consistently for an extended period so as to reinforce the concepts.
2) Eucharistic Prayer I is really long. And I say this not out of laziness but as an objective fact. It is probably 7-8 minutes longer than Eucharistic Prayer II. If you are going to encourage people to prayerfully reflect over the concepts of a particular Eucharistic Prayer, it seems to make sense to start with the shortest of them, which is Eucharistic Prayer II.
I also think the length is important because you need to build up to a more lengthy prayer.
We live in a culture where, let's be honest, one of the most notable things people point to when talking about Mass at various parishes is the length of time Mass takes at that parish.
I'm not saying we cater to that, but I do think we need to recognize that to the AVERAGE people in the pew today, that's the most important thing, so why not start with a shorter Mass and, over time, build up to a more lengthy Eucharistic Prayer? It isn't like Eucharistic Prayer II is invalid, it is a prayer of the Church too, so it isn't like Eucharistic Prayer II is a "settling for less" anyway. Thinking that way (as I did as a younger priest) is very dangerous and wrong.
3) Eucharistic Prayer I was written to be prayed silently (and thus much faster). For hundreds of years, Eucharistic Prayer I was the ONLY option for priests. In the Traditional Latin Mass the "Eucharistic Prayer" was prayed silently by the priest to allow people to be praying in the midst of silence. (As a side note, as our world gets louder, I think people would love this option today - priests pray the Eucharistic Prayer silently so we can have some more silence at Mass).
If you have ever prayed a rosary silently vs. out loud, you realize things get prayed faster silently - and "faster" doesn't necessarily imply "more irreverently" at all - in fact we've probably all been somewhere where someone was praying TOO slow, to the point that the SLOWNESS was a distraction. That prayer is quicker when prayed silently does not have any implications for the prayer's efficacy or reverence.
I pray Mass in the older form and obviously in the new form, and Eucharistic Prayer II prayed out loud is about the same length as Eucharistic Prayer I prayed silently.
In summary, I plan to build up to Eucharistic Prayer I, but it is a multi-year process. Even when I get to the point where I would pray Eucharistic Prayer I, I would still rotate among the three prayers as they are all beautiful and have different phrases and concepts that we need to be reminded of.