I'm not sure I understand what you mean about keeping the block time. If there is none or very little mining, that would be out of your hands. When we got stuck with the high difficulty, it took hours to move one block. If miners go away due to the coin being mined out, you might not be able to set the difficulty low enough to counter-act a near lack of miners. Blocks would move slower than a 7 year itch.
I don't believe PoS would help much in that regard, either. Suppose people hold coins in their wallet for staking but they simply close their wallet while they wait (I do exactly that with CPTL, CRYPT, RDD, and a few others). I don't keep my PoS wallets open for days waiting to see the payout and I'm pretty sure others don't. In addition, I've lost many coins due to PoS not working properly, so I don't stake any of the coins I have that might be worth anything at all sometime down the road. I only stake the coins I really don't care about losing.
Not sure exactly how it works. But while staking the coins that I do in fact stake, my CPU does some kind of work. Even with an 8 core clocked up to 5.9 GHz per core, it is hella slower than a single R9 270 GPU mining a pool. And the R9 is hella slower than the smallest ASIC in my arsenal. A large number of people doing PoS would have to keep their wallet open in order to make the same progress as one amateur miner with a handful of Gridseeds.
At this time, I have many of my rigs available for rent, when they are not rented, each rig will hash to a pool of my choosing, and each rig can point to a different pool.
BUT
I always leave 100 MHs worth of smaller ASICs not up for rent and pointed at a couple of different LTCD pools. If/When mining LTCD stops, there's not much I can do to help transaction speed. I'm sure there's at least a couple other people out there with rigs doing the same thing for you. Keep the mining going as long as possible with a low payout (enough to cover electric cost) and you can expect transaction transfers to remain decently quick.
I'm pretty sure we keep the block time to keep the network fast. We would lower the block reward dramatically to keep the coin mineable for a long time and to be able to use POS. We in the DEV team are making the cahnges as fast as the community is finished voting.