There are around 290M cars on the road in the US, of which 1.3M are electric. Over the next 20 years, the majority of those 290M will get replaced by an electric vehicle. One of the major drivers of this will be cost: it will just be significantly cheaper from a TCO perspective to drive electric.
Electric is such a shift that traditional car purchasing forces like brand loyalty are disrupted, and there will be a land rush to claim new market share. Electric also presents heretofore prohibited avenues for vertical integration: Tesla wants to sell you the car, the home battery and the solar panels to go with it. Standard Oil was broken up for similar types of vertical integration.
Electric will also present opportunities for ongoing service revenue (value-added services such as in-car infotainment) that automakers have tried for years to break into (unsuccessfully)
If you’re the early leader with a promising track record, as Tesla is, a 1T valuation is not surprising.