This is the decision that brought about a whole breed of bike, the cyclocross bike (aka gravel bike, adventure bike etc etc). Basically a road bike that can take big chunky tyres and has a higher bottom brackets to help off road - you get a road bike and a mountain bike for the cost of one bike and an extra pair of tyres (wheels as well if you're going to need to make quick changes).
I'll be honest - I bought myself a Cyclocross bike very much on that basis...
Bit of Background...
I've got a MTB. it's nice enough, solid (to the point of being completely over-built to be honest - it was specc'ed when I was around 50kg heavier than I am now, and specc'ed tough enough to cope with my riding around in the Peak District / Lakes) - On-One Inbred 456, 26" wheels, full XT groupset, disks (naturally) and a fairly uninspiring fork at the front. Forks were pretty much "consumable" items when I was at my heaviest - even the £500+ ones were very much "max rider weight of (well under mine)" - so blown seals, broken springs, knackered bushings, you name it. But, I rode, and I kept riding, even riding it locally, around my local trails - which are pretty much either bridleways, canal towpaths or sustrans routes - definitely nothing even slightly "gnarly" or "rad doood"... BUT, lugging this pig-iron 18+kg POS around meant I burned lots of kCals, and I lost weight. Quite a lot of it. And eventually I felt fit enough to risk riding one of my old road bikes. First time back on 21c tyres (yep, 21c - it was a 1990's full-on racing bike, and didn't have clearance for anything wider!) was terrifying, but in a good way - the speed, the freedom, the fact that it actually rolled uphill and didn't need to be "winched". Sold. 1 Modern Roadbike. Bought myself a Planet-X Superlight Alu frame, Ultegra Oily Bits and built myself some 36 spoke wheels (Ultegra Hubs, H-Plus Son Archetype Rims, DT-Swiss Stainless for the geeks) and pretty much didn't look back.
Thing Is, at the time I was a full-time carer to my parents - I basically got a couple of hours in a afternoon where I could go out, either to run errands, OR to get some exercise. So, I'd a choice of driving to the edge of the peaks and getting maybe 30 minutes on the MTB, before having to drive back (late) - OR go and ride "out of the door" - which 2 hours on the roadbike would see me getting a good 25+ miles in. Again, a no-brainer - more riding = more kCals burned = more fat lost.
three years on, and the mechanicals were getting a bit "creaky" - BUT - I'd promised myself if I hit my "first target" weight, I'd treat myself to a new bike. And I did, so I did. Finally felt light enough to risk Carbon. Dolan Tuono SL, Full Ultegra, FSA Team Issue Cockpit, Bloody Lovely. And I pensioned off the Planet-X to the indoor trainer. BUT, I now rode all year round. Bought some cheap wheels for the Dolan, and some Guards. Hated it - like having a greyhound and putting it in Gumboots. So dug the Planet-X our for another winter, and put the Dolan on the trainer instead. The Winter finally pretty much killed the Planet-X's componentry - and as it was 9 speed Ultegra, it was quite an expensive refurb.
Over the summer a plan dawned...
I no longer really rode my MTB on anything that needed anything so "overbuilt". But I did enjoy riding the offroad trails around home - it's a good way to get a hard cardio workout in a hour or so, in the rain... I also needed a "winter hack" bike - something that would happily take mudguards, 28c tyres - ideally would have disc brakes so it'd stop in the rain. Yep - I decided on a "not really very racy" Cyclocross Bike. Cannondale CaadX 105 Disc. Its got discs, it's got 2 bottle cages, mudguard and rack eyes, so I could use it as a light tourer if I wanted with a pair of tough 28c road tyres on it. Or, with a pair of 'Cross knobblies fitted, It's light enough (just about) to race if I wanted, it handles pretty well offroad, and its tough enough to cope with any of the non-gnar offroad that's on the menu in anything in my "out of the door" rides. I'm no longer performing the carer role (they've both sadly passed by now) but I still find myself being pretty heavily averse to wasting 1 hour driving to somewhere worthwhile to ride the MTB, only to be sick of the sight of the MTB within 40 minutes because it is just so heavy and slow and bloody miserable to ride. Then having to drive a hour back home, and another hour or so to clean the bike, and the car, and every stitch of clothing I was wearing.
If I lived In the Lakes, or the Welsh Hills, Or Scotland, or even the Peaks, then I'd probably think differently, but for me, at the moment, the MTB is overkill... a 'cross bike is where it is at for me for "mixed terrain" (road and off-road) stuff...
Of course, if someone's got a nice freebie Carbon 8kg Full-Suspension 29er they want to give me on long-term loan to convince me that MTB's still have their uses, I'm amenable...