People toured for decades on only a double front and a five cog rear cassette.
That's right and they also toured on a single fixed gear or even on a high wheeler because that's all that was available. We don't have to. There has been progress in equipment and it's foolish not to take advantage of it.
A ten-speed cassette requires a ten-speed deraileur,......
That's wrong. A 9-speed rear derailleur works fine on a 10-speed cassette. The "speeds" are in the shifter, not the rear derailleur. You are correct that a 10-speed drivetrain does require a 10-speed shifter, cassette and chain.
One problem the OP will have is that Shimano and SRAM don't make 10-speed MTB range cassettes, only 9-speed so if he wants to go to a Xx32 or Xx34 cassette he will need a 9-speed. IRD does make 10-speed MTB range cassettes but I've heard mixed reports about their quality. One "cure" for having 10-speed brifters and wanting to use 9-speed everything else is Jtek's "Shiftmate". These things are relatively cheap (~$35) and work wonderfully well.
And know that in many cases, switching to 10-speed transmissions gains nothing on the extremes, you've just divided the intervening options into finer slices. If you add a huge cog and a tiny chainwheel, you must swap out the mechs to accommodate the huge swings. The lang cage rear mech has the ability to suck up all that extra chain when you drop into the lowest granny or ratchet up to the tallest gear.
True, road 10-speed cassettes tend to provide more intermediate cogs instead of a wider range.
A long cage MTB rear derailleur is required on wide range cassettes for two reasons:
1. To provide the chain wrap needed to accomodate the wide range of cog and chainring sizes.
2. To clear the large cog on a MTB cassette. Shimano road derailleurs are rated to clear a 27T maximum cog. They will usually accept up to a 30T but a 32 is very iffy and a 34 almost never works. An MTB rd is made to work with these wide range cassettes.