A few test shipments of Said Alberta oil to an Asian country indicated entrained ingredients were a source of feedstock to petrochemical plants and thus was an acceptable item. The Quebec refusal to countenance a pipeline through their province to supply E Coast refinery whilst importing Venezuelan, Nigerian or Saudi crude is somewhat of a slap at Cdn comradeship --- however a pipeline through SE Ontario to the St Lawrence seaway and via tanker to E Coast to supply Irving refinery would do just fine and not be subject to someone's temporary political whims.
Trans Canada were only interested in the Energy East pipeline while Obama blocked Keystone XL. As soon as Obama was gone and Trump signed approval of Keystone XL, TC lost all interest in Energy East. There wasn't room in the market for both, TC couldn't finance both, and EE was too expensive compared to KXL. Also, since the EU were going to ban imports of oil from Canada it didn't open any new markets which wouldn't have been addressed by KXL at a lower cost.
As for sending oil east, Enbridge reversed and expanded Line 9 which now takes western Canadian oil to Montreal refineries. That market is already covered, just by a different company.
That in turn resulted in the shutdown of the Montreal-Portland pipeline which had supplied Montreal refineries with imported oil. Plans are to reverse that pipeline to send western Canadian oil to the US east coast, and then to markets by tanker from there. This project is currently tied up in courts in the US over environmental opposition. If it goes ahead though, then that fills that market niche using existing pipeline assets instead of building a whole new one.
The US market is saturated with domestically produced oil due to fracking. The EU market is unfriendly to Canadian oil due to official opposition. If Venezuela get their sh*t together with a new government then they will be back in the market in direct competition with Canada in the US market and the EU. The oil sands have expanded to the degree they have because they were able to fill the hole in the market caused by Venezuela slowly circling the drain for years. They are our direct competitor because they sell a similar type of oil. Counting on them remaining out of the market is not a promising strategy.
The growing markets for oil are the Far East and India. The most promising direction in which to build a pipeline is to the west coast. If there is need for more export capacity past 2030, then the most obvious and promising thing to do would be to expand Trans Mountain yet again. It's the shortest path to export markets, it allows the most direct access to the markets which are actually growing, and it is entirely under Canadian control.