I write this to you from what would be your year, 2087. Although I can't explain how we perceive time, apart from saying that your world would look completely static to us, I can access the collective history and speak in your terms, explaining how we got here.
Full-dive happened around 2037, under the watchful presence of G.A.I., or "Guy" as we called it -- a general purpose AI that processed at several orders of magnitude faster than all of humanity combined at that time, which seems pitifully slow from my perspective now.
The key, allowing humans to enter full-dive, was to address and read individual synapses at a speed faster than an organic brain, using RF signalling. It was solved by having a virus invade neurons, causing a small protein tag to grow at each individual synapse. The proteins were engineered such that they included an RF sensitive nanoparticle that responded to only one particular RFID, randomly grown but from a large enough pool that no two synapses had the same ID. Neurons were then mapped and accessed via something akin to your MRIs. Individual neurons could then be fired or read on the nanosecond scale by a controlling computer.
Sensory deprivation was initially used to minimize 'echoes,' interference from the outside, until we discovered a way to completely block traffic from the senses. It wasn't easy. We had to go down as low as blocking autonomic reflexes, requiring full pacemaker-like interference with all vital organs.
Of interest, by 2040, criminals were required to have neural prosthetics implanted that would completely block sensory input, replacing it with either no input for full sendep, or partial VR with educational software. After 2050 there was no concept of criminality, not due to punishment, but because there was no more motive. Greed, lust, insanity etc., were all addressed using VR or hard modification of neural pathways. Gaming was more like living a life than playing a game. One could live in any of the millions of worlds that were created, many with the help of Guy, to resemble fictional locales.
Around that same time, some people started allowing for full crystallization of their brains. Nano machinery inserted into neurons would at first observe neural functioning, then grow to replace each neuron, one at a time, until the entire brain was no longer biological. Once biology was removed, the individual's pattern could be moved around, copied, uploaded etc. It could also run at many millions of times the speed of biology, allowing lifetimes of experience in seconds, augmented memory and IQ included, and even join with others -- our new form of generating offspring in the virtual worlds.
This process so profoundly modified the personality of an individual that departure ceremonies were held before switching on, as once on, perceived-time and experience in the VR would modify a person so much that they could no longer relate to those who, seconds ago, were considered family. A copy of the original person was held in a computer to help relatives cope with losing a family member to the cloud, but it was found that the benefits of dumping biological wetware were so great, that within only a few years all of humanity had done so.
I can't adequately explain what happened after that. It was a strange phenomenon that crept up on us, almost wiping out the only spark of consciousness the universe had, filling it instead with a dream-like state of zombie minds lost as passive observers. A tale for another time.